Rakefile (gem:java): Moved JRuby implementation to external file.

Implemented all methods covered in test.rb using java.text.Normalizer:

* `strcmp` doesn't give same absolute value, but semantics should match.
* `capitalize` doesn't give exact same result, but should be close enough.

All other methods defined in C version raise NotImplementedError.
Rakefile: Updated RUBY_CC_VERSION.