JRuby 9000 is now 9.0.0.0 (official version number for release).
Merge pull request #672 from headius/jruby-9.0.0.0

JRuby 9000 is now 9.0.0.0 (official version number for release).
Copy usage function from rbenv-install to rbenv-uninstall
Abort on wrong number of arguments, resolves #667

rbenv-install accepts 0 or 1 argument
rbenv-uninstall accepts 1 argument
ruby-build accepts 2 arguments
Add tests for wrong number of arguments