Remove therubyracer because of the following reasons:

* therubyracer is unstable between platforms, platform versions, ruby interpreters and even ruby versons.
  Pretty much everytime there's native extension compilation issue, it is therubyracer.
* therubyracer is officially discouraged by heroku as it uses lot of memory. On the other hand node.js advertises itself as very lightweight.
* using therubyracer on server is just like installing Node that can be used only by ruby apps (you're using v8 anyway)
* btw. node.js is necessary only on Windows and Ubuntu development, on Mac development ExecJS is using JavaScriptCore.

catched from https://github.com/sheerun/rails4-bootstrap/issues/18