Don't try to save PNG as CMYK

A JPEG image could have mode CMYK but this mode is not allowed
for PNGs. Using filters:format(png) with a CMYK JPEG image will
raise: `IOError: cannot write mode CMYK as PNG`
Merge branch 'master' of https://github.com/RealGeeks/thumbor
Added a test for @igorsobreira pull request #234