jruby 9.3 fixes

[ahorek]

Overview

this PR fixes compatibility with jruby 9.3+

usually, programs don't need to access JRuby.runtime directly, so it's no longer available automatically since jruby 9.3

for special cases like accessing object space, we need to load the runtime explicitly. This change is backward compatible.

Related Github Issue

#788 #784

Testing

the current CI on this project is more complex than I originally thought, many tested libraries are very outdated, rails 3 doesn't work well on jruby 9.3 anymore and I think there's one new bug in jruby's autoload... so CI switch should be done separately.

Reviewer Checklist

• Perform code review
• Add performance label
• Perform appropriate level of performance testing
• Confirm all checks passed
• Add version label prior to acceptance

[CLAassistant]

All committers have signed the CLA.

[kaylareopelle]

Thank you for submitting the fix in this PR and the others for JRuby support! Our team will review the PRs today and get back to you with any suggestions.

[kaylareopelle]

Thanks for explaining the reasoning behind this change in the description of the PR!