Missing coffee-rails dependency in gemspec.

[babatakao]

This gem uses CoffeeScript, but the gemspec lacks coffee-rails dependency.

[mwlang]

coffee-rails dependency added. I incorrectly assumed at outset Rails projects using this would also always use coffee-rails. Although I've added the dependency and released as 0.1.10 just now, the smarter route may be to remove *.coffee and just use *.js asset files as coffee ends up not being overly useful in any major way in this project.

I didn't have time to remove coffee dependency, so added the gem dependency for now, but leaving this issue open to anyone who wants to convert the coffee scripts to pure JS scripts and submit a PR.