tightens up the haml dependeny in the gemspec (disallows haml 4)

Hi Brian,

I'm convinced that if you were to trigger the travis hooks in master that all the tests would fail, as haml 4 will be brought in, instead of the 3.1 series.

This is the reason my other pull requeest https://github.com/spree/deface/pull/78 is failing.

This took me a while to figure out as I'm not a gemspec guru (yet), but I'm fairly certain this is the case.

In this commit, I have tightened up the constraint to keep us < haml 4.

Haml 4 gives us a railtie error, at least in the specs (it is easy enough to reproduce as I'm sure you are aware).