Add a JRuby-based ext wrapping the net.jpountz.lz4 library.

This PR adds support for JRuby to the lz4-ruby gem. The C ext has been replaced with a JRuby ext that wraps the net.jpountz.lz4 library, and I've made necessary modifications to the Rakefile to build the JRuby gem with ```rake build:jruby```.

The JRuby version of the gem passes all tests.