Occasionally failed by "JRuby runtime is not loaded successfully" error

# Summary
The Embulk process occationally fails due to following error when doing input-file and output-postgresql.
```
2018-03-08 03:02:53.999 +0900 [INFO] (main): Started Embulk v0.9.4
2018-03-08 03:02:54.477 +0900 [INFO] (0001:transaction): Gem's home and path are set by default: "/root/.embulk/lib/gems"
SystemCallError: Unknown error (SystemCallError) - channel: org.jruby.util.io.ChannelFD@1da5c19d uri:classloader:/META-INF/jruby.home/lib/ruby/gems/shared/specifications/default/minitest-5.4.1.gemspec
gets at org/jruby/RubyIO.java:2301
readline at org/jruby/RubyIO.java:2692
block in data at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:118
open at org/jruby/RubyIO.java:1156
open at org/jruby/RubyKernel.java:306
data at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:116
valid? at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:207
select at org/jruby/RubyArray.java:2566
gemspec_stubs_in at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:751
default_stubs at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:758
stubs at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:833
find_by_path at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:1038
try_activate at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems.rb:209
require at uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:126
at :1
org.embulk.jruby.JRubyNotLoadedException: JRuby runtime is not loaded successfully.
at org.embulk.jruby.ScriptingContainerDelegateImpl.runScriptlet(ScriptingContainerDelegateImpl.java:725)
at org.embulk.jruby.JRubyInitializer.initialize(JRubyInitializer.java:128)
at org.embulk.jruby.LazyScriptingContainerDelegate.getInitialized(LazyScriptingContainerDelegate.java:206)
at org.embulk.jruby.LazyScriptingContainerDelegate.runScriptlet(LazyScriptingContainerDelegate.java:186)
at org.embulk.jruby.JRubyPluginSource.newPlugin(JRubyPluginSource.java:60)
at org.embulk.plugin.PluginManager.newPluginWithoutWrapper(PluginManager.java:59)
at org.embulk.plugin.PluginManager.newPlugin(PluginManager.java:31)
at org.embulk.spi.ExecSession.newPlugin(ExecSession.java:147)
at org.embulk.spi.Exec.newPlugin(Exec.java:63)
at org.embulk.exec.BulkLoader$ProcessPluginSet.<init>(BulkLoader.java:410)
at org.embulk.exec.BulkLoader.doRun(BulkLoader.java:484)
at org.embulk.exec.BulkLoader.access$000(BulkLoader.java:35)
at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:353)
at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:350)
at org.embulk.spi.Exec.doWith(Exec.java:22)
at org.embulk.exec.BulkLoader.run(BulkLoader.java:350)
at org.embulk.EmbulkEmbed.run(EmbulkEmbed.java:162)
at org.embulk.EmbulkRunner.runInternal(EmbulkRunner.java:292)
at org.embulk.EmbulkRunner.run(EmbulkRunner.java:156)
at org.embulk.cli.EmbulkRun.runSubcommand(EmbulkRun.java:437)
at org.embulk.cli.EmbulkRun.run(EmbulkRun.java:92)
at org.embulk.cli.Main.main(Main.java:26)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.embulk.jruby.ScriptingContainerDelegateImpl.runScriptlet(ScriptingContainerDelegateImpl.java:718)
... 21 more
Caused by: org.jruby.embed.EvalFailedException: (SystemCallError) Unknown error (SystemCallError) - channel: org.jruby.util.io.ChannelFD@1da5c19d uri:classloader:/META-INF/jruby.home/lib/ruby/gems/shared/specifications/default/minitest-5.4.1.gemspec
at org.jruby.embed.internal.EmbedEvalUnitImpl.run(EmbedEvalUnitImpl.java:131)
at org.jruby.embed.ScriptingContainer.runUnit(ScriptingContainer.java:1307)
at org.jruby.embed.ScriptingContainer.runScriptlet(ScriptingContainer.java:1300)
... 26 more
Caused by: org.jruby.exceptions.RaiseException: (SystemCallError) Unknown error (SystemCallError) - channel: org.jruby.util.io.ChannelFD@1da5c19d uri:classloader:/META-INF/jruby.home/lib/ruby/gems/shared/specifications/default/minitest-5.4.1.gemspec
at org.jruby.RubyIO.gets(org/jruby/RubyIO.java:2301)
at org.jruby.RubyIO.readline(org/jruby/RubyIO.java:2692)
at RUBY.block in data(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:118)
at org.jruby.RubyIO.open(org/jruby/RubyIO.java:1156)
at org.jruby.RubyKernel.open(org/jruby/RubyKernel.java:306)
at RUBY.data(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:116)
at RUBY.valid?(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/stub_specification.rb:207)
at org.jruby.RubyArray.select(org/jruby/RubyArray.java:2566)
at RUBY.gemspec_stubs_in(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:751)
at RUBY.default_stubs(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:758)
at RUBY.stubs(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:833)
at RUBY.find_by_path(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/specification.rb:1038)
at RUBY.try_activate(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems.rb:209)
at RUBY.require(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:126)
at RUBY.<main>(<script>:1)
Error: JRuby runtime is not loaded successfully.
2018-03-08 03:02:54.857 +0900: Embulk v0.9.4
```

# Goal
Figure out the cause and resolution or workaround.

# Environment information
EC2: t2.medium
OS: CentOS 7.4
Java: OpenJDK 1.8.0
PostgreSQL: 9.6.7
embulk: 0.9.4

# Situation
Our workflow over view is:
1. Data import from csv file to local postgresql server. (Embulk + input-file + output-postgresql)
2. Data analizing in postgresql.
3. Data export from postgresql to csv file. (Embulk + input-postgresql + output-file)

Above process is looped every csv file by shell script.
The error happens at step 1. And it seems happen when importing slightly larger file (200MB or 500MB).