upgrading json and multi_json gem versions on which tinder depends. Also modified specs to remove deprecation warinings

Since Json gem version 1.7.7 has become a base version for most common usage, it is necessary to upgrade the version of json gem that tinder depends on. Since the current version is 1.8, am changing it to the most recent version. Not sure if this warrents a minor version number upgrade. but have changed the version number to 1.9.3 anyways!