force rgba colour (6) since iOS does not support indexed (3)

Some of the images have no transparency and were thus being optimised
to indexed palettes which iOS does not support. This change forces
colour type 6, which corresponds to rgba.

See:
http://www.w3.org/TR/PNG-Chunks.html
http://en.wikipedia.org/wiki/Portable_Network_Graphics#Color_depth
Merge pull request #4 from guardian/force-rgba-colour-encoding

force rgba colour since iOS does not support indexed pngs