Extend Foodcritic compatibility to include 3.x

Foodcritic v3.0.0 shouldn't have any backwards incompatibilities in the CLI ([changelog](https://github.com/acrmp/foodcritic/blob/master/CHANGELOG.md#300-14th-september-2013)), so it should be safe to loosen the dependency requirement to include 3.x.

Tests pass and everything seems to work as with previous version.

Fixes #3.