Remove dependency on jruby-gradle plugin

[robertpanzer]

It seems like the jruby-gradle plugin is no longer maintained.
It doesn't work with the latest version of Gradle, it doesn't resolve anymore with the default gradle plugin syntax, and there hasn't been any activity on the repository for more than a year.
Corresponding issues in jruby-gradle/jruby-gradle-plugin#452 and jruby-gradle/jruby-gradle-plugin#451 are also unanswered.

I think the only functionality that we use today from this plugin is the option to install Gems.
It should be possible to invoke JRuby ourselves to install Gems in our build script.

I think the only dependencies that AsciidoctorJ should have is JRuby and of course the ruby gems.