Ignore directories when recursively scanning

The call to `Find.find` was already recursing, but it was adding
directory names onto the `files` list, which in turn was causing the
Sass engine to choke trying to load a directory as a file.

Fixes #4.

Change-Id: I7c59ea3ff08410602d9e85459d8d615b49be9cb0
Reviewed-on: https://gerrit.causes.com/19252
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>