Removed extraneous whitespace.'
Deletes *.fail.png files for tests that no longer fail.

If a test once failed, but now passes, this change deletes the stray
fail file that hangs around in the screenshots dir.

To use, add the following to your config.js options:

    deleteInvalidFails: true

This option is only useful if cleanupComparisonImages is false. When
cleanupComparisonImages is true, that'll take care of the deletion.