Move adapter dependencies to gemspec

I think that this gem would provide a lot more utility if it automatically
installed the appropriate adapter. For instance, I use Linux. I want rb-inotify
installed automatically. This happens if you clone the repository and `bundle
install`, but not if you install the gem. The gemspec should contain the
adapters--not the Gemfile. The Gemfile should be for project development, and
the gemspec should tell projects utilizing the gem what its requirements are.
See [Clarifying the Roles of the .gemspec and Gemfile][wycats article] by
@wycats.

The side effect is that I have to put `gem 'rb-inotify', '~>0.8.8'` in my own
Gemfile so it will be available to guard. Since listen is the only gem that uses
it, I think it's incorrect for me to be responsible for maintaining the
dependency.

Thanks! :heart: guard!

Justin

[wycats article]:http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/