Enable "promoteImage :: PixelRGB8 -> PixelRGBA16"

Hi,

I wanted to be able to promote an "Image PixelRGB8" to an "Image PixelRGBA16", therefore I have made the following change. 

I also lowered the version number of two dependencies to be able to compile this package on a Debian 7 system.

I think this changes might be useful also for others.

Kind regards,

Kai