show definitions in proper sort order - by version number

Previously, jruby versions would show as follows:

jruby-1.7.1
jruby-1.7.10
jruby-1.7.2
jruby-1.7.3

After this change, they show as:

jruby-1.7.7
jruby-1.7.8
jruby-1.7.9
jruby-1.7.10

I'm not sure if the version on Mac OS X supports this flag. It works on Ubuntu 13.10