remove runtime dependency on rails 4

rails 4 is a massive runtime dependency for a small utility. I skimmed the code and it seems to run fine without it.

Extra info:
I'm also considering adding support for descriptions: https://github.com/google/closure-templates/blob/master/examples/examples_translated_x-zz.xlf#L8
but after some more discussion we might not need this.
Thanks for the gem! I'm using it on https://translate.twitter.com now.