Update `therubyracer` gem

The older version of `therubyracer` gem, which jshint depends on, is not compatible with OSX Mavericks and will cause compilation issues.

Please reference too: http://stackoverflow.com/questions/19630154/gem-install-therubyracer-v-0-10-2-on-osx-mavericks-not-installing 

PS. All tests are still passing :)  Thanks for the great gem.