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).
Merge pull request #42 from opscode-cookbooks/jtimberman/fix-file-existence-for-module

Fixes #34, closes #37