Bump sass dependency to >= 3.3.7

The issue reported in https://github.com/causes/scss-lint/issues/151 was
fixed and merged into the upstream sass repo, landing in 3.3.7. Bump our
version requirements to load at least this version.

Change-Id: I46eca08e7878827c42e0ae36546505b8e9248945
Reviewed-on: http://gerrit.causes.com/40475
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>