Bugfix/therubyracer only on linux

Since @steveklabnik suggests therubyracer in the Gemfile causes rstat.us not to bundle on OS X, I will simply remove it as a dependency on OS X and Windows where it is not necessary anyway.

However, I can confirm that on my Linux machine, not having therubyracer will indeed cause rstat.us to not be able to generate assets. It fails when I comment it out... works when I remove the comment and bundle. It is still necessary.

This makes sense. You need a js engine to use coffeescript, etc. On OS X, it uses the one provided with Safari. On Windows, the one used for IE can be used. On Linux, however, it does not have a guaranteed engine installed. And since rstat.us should be viable for all deployment opportunities, this means it will find itself on bare Linux environments. It should not hurt to have therubyracer installed even if libv8 is installed through different means on Linux.

Therefore, the Gemfile contains a condition that checks the platform for OS specific tags. It only blacklists the two OSes, so it should be rather conservative.