Fail back to 'json' gem if 'multi_json' is not available

Newer version of `fog` which requires `multi_json` gem don't work on if a user uses an older version of RubyGem, because `multi_json` requires a newer version (>= 1.3.6). Good example of this is Ubuntu 10.04 where the latest RubyGem version available in the official APT repo is `1.3.5`.

My solution is probably not the best and the final one, but I just want to start the discussion so we can figure out a best solution for this together.