Build Failure: Could not verify the SSL certificate for https://rubygems.org/quick/Marshal.4.8/logstash-devutils-1.3.3-java.gemspec.rz.

Jenkins builds are intermittently seeing the following error:

```
16:42:23 Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF8
16:42:29 Error Bundler::InstallError, retrying 1/10
16:42:29 An error occurred while installing logstash-core (5.6.1), and Bundler cannot continue.
16:42:29 Make sure that `gem install logstash-core -v '5.6.1'` succeeds before bundling.
16:42:32 WARNING: can not set Session#timeout=(0) no session context
16:42:46 Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF8
16:42:52 Error Bundler::InstallError, retrying 2/10
16:42:52 An error occurred while installing logstash-core (5.6.1), and Bundler cannot continue.
16:42:52 Make sure that `gem install logstash-core -v '5.6.1'` succeeds before bundling.
16:42:54 WARNING: can not set Session#timeout=(300) no session context
16:43:04 WARNING: can not set Session#timeout=(0) no session context
16:43:04 Error Bundler::Fetcher::CertificateFailureError, retrying 3/10
16:43:04 Could not verify the SSL certificate for https://rubygems.org/quick/Marshal.4.8/logstash-devutils-1.3.3-java.gemspec.rz.
16:43:04 There is a chance you are experiencing a man-in-the-middle attack, but most likely your system doesn't have the CA certificates needed for verification. For information about OpenSSL certificates, see bit.ly/ruby-ssl. To connect without using SSL, edit your Gemfile sources and change 'https' to 'http'.
```

Example: https://logstash-ci.elastic.co/job/elastic+logstash+5.6+multijob-intake/61/console