fix json gem issue #163

Fixing issue #163 but locking httparty dependency to `0.11.0` so that json `~> 1.7.6` can still work.

Also since we're still supporting Ruby 1.8.7 i've changed our dependency on the mime-type gem to `~> 1.2.5`.  Recently mine-types [2.0](http://rubygems.org/gems/mime-types/versions/2.0) was released (28th Oct) which requires >= Ruby 1.9.3.