Guard::Guard deprecated in favor of Guard::Plugin

Replaced deprecated Guard::Guard with Guard::Plugin, as instructed here: https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#changes-in-guardguard

I'm still bit new to Ruby, so please let me know if you require more changes before accepting this PR. I suspect there will be questions around Guard dependancies, and release versions…

Thanks,

Paul