Upgrade for Guard v2 API

This is a very quick application of the upgrade instructions on the Guard Wiki here:

https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0

* change inheritance to be Guard::Plugin instead of Guard::Guard
* remove watchers array from constructor call in specs
* make runtime dependency on guard >= 2.0, <= 3.0

I didn't add any new tests, but all existing specs pass.