fixes #20 patch pHash to support alpha channel PNGs

After a lengthy dicussion on https://github.com/mperham/phashion/issues/20, I'm suggesting, 
* merge and release the gem with this custom patched version of pHash
* re-release once pHash pulls in patch version and move back to official pHash codebase

The `ext/phashion_ext/pHash-0.9.6-png-patch.tar.gz` contains these changes, https://github.com/westonplatter/phash/commit/ff255d2d3f93c841b98923ecbde997027f21ae36