Fixes #34, closes #37

The `creates` attribute would get an incorrect value for the
`module_path` attribute to check its existence. The magic that is done in the
helper method sets the attribute to a different value - the actual installed
module location - in the `mod_rails` recipe.

This change removes the `creates` attribute and uses the later-evaluated value
of `module_path` to check its existence. This is implemented as a separate
`not_if` than the `only_if` to make it easier to read (and have a specific
comment about).