Adds support for HAML 4

This is a simple PR that just allows HAML to be anything greater than the current gemspec's `~> 3.1`. 

Feel free to use `~> 4.0`, but with the extremely limited usage of HAML in mailcatcher, I just don't see an update to HAML ever breaking this gem, and we don't really need a new release just to keep in sync with HAML.

As it is, our usage of mailcatcher is broken because the last version that doesn't have a HAML requirement is 0.2.4, which doesn't support the `-f` flag.