Loosen `listen` gem dependency

See the commit message for details. Guard makes use of some new methods in the `listen` gem. In order to use both 1.6.x of `guard` and `compass` we need to loosen the dependencies a bit on versions.

Ended up just doing a range (it may work with latest listen gem `0.7.x` but did not want to mess testing every version and instead just fix the specific issue.

`gem.add_dependency 'listen', ['>= 0.5.3', '<= 0.7.0']` is what the line now looks like.