Make gemspec target MRI and JRuby platforms separately

[jsvd]

the MRI version of the gem shouldn't have JRubyFileExtension.jar included in it, and the jruby version should be correctly target the "java" platform.

[wiibaa]

From #37 and the fact that windows support was added thanks to JRuby extension, I suppose you need to make it clear somehow that currently MRI gem does not work properly on windows