Upgrade Haml to prevent it from mutating our strings.

https://bugzilla.redhat.com/show_bug.cgi?id=1134863

Haml 4.0.3 was doing gsub! on provided strings and was broken with frozen Strings.  It was fixed in 4.0.4 and 4.0.5 in this commit: https://github.com/haml/haml/commit/5bb46925b2b5211c7cb2052b8772872689e25346

Note: I had to run `bundle update haml brakeman` to get upgraded to 4.0.5 haml locally.
Merge pull request #503 from jrafanie/upgrade_haml_to_not_mutate_strings

Upgrade Haml to prevent it from mutating our strings.