Support RUBY_ENGINE_VERSION

[postmodern]

Rubinius has added support for RUBY_ENGINE_VERSION to help ruby switchers distinguish between the ruby version (1.9.3) and the implementation version (1.7.9). I think JRuby would also benefit from exposing this constant.

[postmodern]

See also postmodern/chruby#329

[havenwood]

It would be really nice to have a cross-implementation, standard way of checking the JRUBY_VERSION equivalent. 👍

[headius]

Seems like a good idea to me...RUBY_ENGINE + RUBY_ENGINE_VERSION is like JVM's java.vm.name and java.vm.version.

[enebo]

I did not notice this was created yesterday and made my own this morning (and then fixed it): #2751

Closing as dup.