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.

See also: https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#changes-in-guardguard
Merge pull request #16 from cristoper/deprecation-fix

Update deprecated code (since Guard 2.0)