The 'ignore_path' was updated to 'ignore-path' in 0.5

When 0.5 was released this option was normalize to match
the rest of the options (which don't use underscores), so
to ensure this continues operating correctly we need to
reflect that change here.

Other changes in 0.5 (not relevant but will list them here):

666805d Fix the 'ignore-path' config option
04a710c Allow overriding file encoding
5a43417 Add check D005 - no newline at end of file

Change-Id: Id1d1b0dc275fc17333d6bd4ef2aa24f114ff7872