Lower the minimum version of criterion and primitive.

In Debian 7 only this older versions are available. The module compiles
fine with this older versions.
Add ColorConvertible Instances to PixelRGBA?[8|16]

Now it is possible to:
promoteImage :: Image PixelRGB8 -> Image PixelRGBA16
Merge pull request #67 from KaiHa/master

Enable "promoteImage :: PixelRGB8 -> PixelRGBA16"