lib/cell/haml_support_that_SUCKS, should make sure user's project defined?(is_haml?)

Cells 4.0 can't work when user's project don't use haml, and it will shows no method error `is_haml?`.
I added a check to make sure they have `is_haml?` method in `#output_buffer_with_haml` and `#set_output_buffer_with_haml`.

It's about Gemfile list, and I have no idea about how to write test… sorry for this.