use guard-compat to fix broken tests

- uses guard-compat >= 0.1.1
- allows Guard::Notify to be used by guard-minitest for tests to run (Guard::Notify will be extracted to a separate gem at some point)
- 'guard/notify' should be included before 'guard/compat/test/helper', because of bad requires inside Guard 
- dependency on Guard >= 2.0.0 should be sufficient (other deps will be handled by guard-compat if necessary)
- `bundle exec rake test` now should work without issues