Convert all 5-digit permissions to quoted strings

This patch converts all autofills for 5-digit integer permissions into quoted 4-digit strings, negating pull #4.

Arguments:
- Ruby 2.0 no longer automatically converts integers with leading zeroes to octals.
- FC006 now supports both methods in the documentation (http://acrmp.github.io/foodcritic/#FC006).
- "0600" is much closer to what an end-user would expect to see on the file system, and is the path of least surprise.