Use my fork of the listen gem for now

I think that guard/listen incorrectly utilizes Gemfile and gemspec, so I'm going
to use my own implementation for now which correctly handles adapter
dependencies. We'll switch back if they merge my [pull request][].

[pull request]:https://github.com/guard/listen/pull/81
Merge pull request #40 from SmartReceipt/update_gems

Use my fork of the listen gem for now