Move project to JRuby

It's important we're moving to JRuby instead of pure Ruby. Reasons:

• It will be easier for users to install JTalks apps. Right now they would need to install both Ruby & Java envs.
• The process of deployment and configuration will be easier and thus it will be possible to use the same Deployment Pipeline as other projects use.

This is not an easy-to-implement change, e.g. JRuby cannot use native extensions and thus they should be replaced with Java libs or Ruby libs that do not use such extensions.