Upgrade a dependency on json library to `~> 1.7.7`.

There's a security issue in json library whose version vagrant requires (`~> 1.6.6`).

https://github.com/ruby/ruby/commit/062d2ee6f798205c3046730d0d348cfd0d0bc09d

This dependency on old version of json library prevents users from installing vagrant with Ruby 2.0.0-p0. This pull request should be merged as soon as possible, I think.