Move therubyracer to production group

It's always been a pain in the ass to build `therubyracer` and `libv8`. Not only do we have many difficulties to build successfully (it fails on OS X 10.10), it also takes long to build.

If we **only** use it for asset pipeline, then we could remove it if we have a JavaScript runtime such as Node.js installed.

Move it to `production` group for now so it doesn't affect production server.

If we can confirm that:

* do not use it anywhere except asset pipeline
* have JavaScript runtime on production server

Then perhaps we'd happily delete it from `production` group too.