Change logger messages to debug

These are very noisy and not worth outputting in my opinion (and sprockets shares the same opinion: https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/manifest.rb#L213 )

Not being able to find gz versions is perfectly fine since images won't have them, so you always get warnings which seems wrong.
Merge pull request #11 from aaronjensen/patch-1

Change logger messages to debug