Disable OS X on Travis as it slows the build considerably
Disable OS X on Travis as it slows the build considerably

* ~10min for OS X build VS ~2min for Linux build
* RubyCI provides a good platform coverage

lucaspinto/rubyspec,eregon/rubyspec,iliabylich/rubyspec,wied03/rubyspec,wied03/rubyspec,wied03/rubyspec,ruby/spec,sgarciac/spec,BanzaiMan/rubyspec,ruby/rubyspec,nobu/rubyspec,sgarciac/spec,eregon/rubyspec,kidaa/rubyspec,eregon/rubyspec,kachick/rubyspec,ruby/spec,ruby/spec,kachick/rubyspec,BanzaiMan/rubyspec,nobu/rubyspec,iliabylich/rubyspec,lucaspinto/rubyspec,sgarciac/spec,kidaa/rubyspec,ruby/rubyspec,kachick/rubyspec,nobu/rubyspec