Support Guard 2.0+

I have updated the gem for [breaking changes in Guard 2.0](https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0). I have also some other minor changes in my [master branch](https://github.com/jnv/guard-shotgun/commits/master), though those are more opinionated.

Also, it would be great to release the current version of the gem with a dependency on Guard ~> 1.0 for people still using the old version of Guard.