Fuzzy gem dependencies

This pull request does 3 things:

  * Makes all gem dependency versions fuzzy
  * Removes the `Gemfile.lock`
  * Bumps `multi_json` up by a minor version (I guess this could be another pull request)

I originally hit issues when trying to use your gem with the current version of `json_spec` due to version mismatches on `multi_json`. This should fix that specific problem and will make it generally more compatible in the future. Here's some more in-depth advice on the subject if you're interested: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/