Conservatively allow Haml 4.x. #434.

`stacktests` (all) passes with Haml 4.0.1, so it seems to be safe to allow it. This commit allows Haml 4.x along with 3.1.x (there is no Haml 3.2, etc.) but not 5.0+. Haml has switched to semver with the 4.0 line, so allowing 4.x should be safe.

This addresses issue #434.