Use multi_json instead of json gem

By specifying a runtime dependency on `multi_json` instead of the `json` gem, it gives more flexibility to the end-user to use whatever JSON library they prefer (falling back to `json` on Ruby 1.9 and `okjson` on Ruby 1.8). In addition to these two adapters, `multi_json` supports Oj, YAJL, NSJSONSerialization (MacRuby only), and gson.rb (JRuby only). This allows for greater performance and compatibility across different interpreters.

This change has already been applied to `cucumber` in https://github.com/cucumber/cucumber/pull/362.