Reverse compatibility_version and current_version for Darwin
The `compatibility_version` should have an API version and the
`current_version` should have a program version of Ruby, but they have
been reversed and the binary compatibility has never worked.
merge revision(s) 56905: [Backport #12983]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@57212 b2dd03c8-39d4-4d8f-98ff-823fe69b080e