Convert Pathname to string before passing it to haml-lint

Was erroring with haml-lint-0.7.0 and ruby-2.1.1/2.1.2/2.1.3
Worked fine with ruby-2.1.4 and ruby-2.1.5.

Discovered via this issue:
https://github.com/mmozuras/pronto/issues/47
Bump version to 0.3.2
Require haml-lint 0.8.0 instead of 0.7.0
Bump version to 0.3.3