Add missing require to haml-lint executable

Commit 8e571c8 accidentally broke the application due to forgetting to
include `haml_lint/cli`.

We don't want to always include the CLI, so add the require to the
executable.

Change-Id: Ic7429a3bfc652d5b4693610ce69e769818015dc5
Reviewed-on: http://gerrit.causes.com/36229
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>