Can't run plugin tests

According to the plugin guide (https://www.elastic.co/guide/en/logstash/current/_how_to_write_a_logstash_input_plugin.html#_clone_and_test) I should be able to clone a plugin and run the following to test:

bundle exec rspec

However, when I do this I get a range of error. First one was:

$ /opt/logstash/vendor/bundle/jruby/1.9/bin/bundle exec rspecbash: /opt/logstash/vendor/bundle/jruby/1.9/bin/bundle: /home/jenkins/workspace/logstash_create_rpm_artifact_15/jdk/JDK7/label/metal-p: bad interpreter: No such file or directory

It appears the path to JRUBY in bin/bundle was hardcoded to some build path. I fixed that to point to /opt/logstash/vendor/jruby/bin/jruby, however then get this error:

$ GEM_PATH=/opt/logstash/vendor/bundle/jruby/1.9/gems/ /opt/logstash/vendor/bundle/jruby/1.9/bin/bundle exec rspec
Gem::LoadError: Could not find 'bundler' (>= 0) among 15 total gem(s)
Checked in 'GEM_PATH=/opt/logstash/vendor/bundle/jruby/1.9/gems/:/opt/logstash/vendor/jruby/lib/ruby/gems/shared', execute `gem env` for more information
to_specs at /opt/logstash/vendor/jruby/lib/ruby/shared/rubygems/dependency.rb:315
to_spec at /opt/logstash/vendor/jruby/lib/ruby/shared/rubygems/dependency.rb:324
gem at /opt/logstash/vendor/jruby/lib/ruby/shared/rubygems/core_ext/kernel_gem.rb:64
(root) at /opt/logstash/vendor/bundle/jruby/1.9/bin/bundle:22

Yet bundler appears to be part of the logstash install:

$ ls -d /opt/logstash/vendor/bundle/jruby/1.9/gems/bundler-1.9.*
/opt/logstash/vendor/bundle/jruby/1.9/gems/bundler-1.9.10
/opt/logstash/vendor/bundle/jruby/1.9/gems/bundler-1.9.6
/opt/logstash/vendor/bundle/jruby/1.9/gems/bundler-1.9.8
/opt/logstash/vendor/bundle/jruby/1.9/gems/bundler-1.9.9

Is there a way I can get the plugin tests to run? I am using logstash 1.5.2.