Retrieve logstash-core gem's path using inline bundler

[andsel]

Release notes

[rn:skip]

What does this PR do?

Avoid to fork a bundler process to extract logstash-core's path and use the same bundler that wraps the execution of the Rakefile itself.

Why is it important/What is the impact to the user?

All the Rake tasks must run with same JRuby distribution that launched the execution. Without this, it uses the system's Ruby.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [x] run with system's JRuby
• [x] run with not the system's JRuby

How to test this PR locally

Verify that it works with Logstash JRuby distribution:

export LOGSTASH_PATH=/tmp/logstash/ && export LOGSTASH_SOURCE=1
/tmp/logstash/bin/ruby -S bundle install
/tmp/logstash/bin/ruby -S exec rake vendor

Related issues

• Closes #217

[andsel]

Note: 🔴 CI is already present also on the main : https://app.travis-ci.com/github/logstash-plugins/logstash-filter-geoip/builds/267431039