Fixes loading of Guard plugin DSL

[ghost]

With Guard 2.12.3, I had a whole bunch of

09:56:46 - ERROR - Could not load 'guard/flog' or find class Guard::Flog
09:56:46 - ERROR - Error is: cannot load such file -- guard/guard
09:56:46 - ERROR - /.rvm/gems/ruby-2.2.0/gems/guard-flog-0.0.4/lib/guard/flog.rb:2:in `require'

So I fixed it.

See:

• https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#guardguard-deprecated-in-favor-of-guardplugin
• https://github.com/guard/guard/wiki/Create-a-guard