Update deprecated code (since Guard 2.0)

Recent versions of Guard emit a deprecation warning about requiring the deprecated 'guard/guard', inheriting from Guard instead of Plugin, and about the changed signature of #initialize. This patch addresses those issues. (Note: these changes will break compatibility with Guard versions 2.6 and earlier.) See also: https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#changes-in-guardguard