Fix/updates for jruby 9.4

Release notes

Adaptations to run with JRuby 9.4

Explicitly state dependency on rexml. Fixes syntax on tests and bumped new version.

Prepare the plugin to run on JRuby 9.4

[x] My code follows the style guidelines of this project
~~[ ] 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~~