Update to make this use a non-blocking version of multi-json

the version of multi_json this is tied to is very old -- it's probably a good idea to update this. 

I've also added a rake file (from bundler) and made the .rvmrc form be better.

can we update? 

(and i'm happy to make releases as necessarily to this gem; will need rubygems access if so. :)) :+1: