Fuzzy gem dependencies

[vanstee]

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/

[tristandunn]

Finally got around to this in eb22e011a15bece06e7b1b677d846603179b0b1d. Couldn't cleanly merged, but credited in change log.

I kept development dependencies a strict requirement for now as I prefer strict requirements.

[vanstee]

@tristandunn Sweet thanks. No problem!