Removed pngcrush option '-reduce' to make alpha-channels work on greyscale images

Great project! I stubmled upon this;
I have multiple color variants for my icons, but the grey icons keep loosing their alpha-channel after pngcrush did it's thing, leaving grey icons with no alpha, and with a black background.

versions:
ImageMagick 6.8.7-7 Q16 x86_64 2013-11-27
pngcrush 1.7.60, uses libpng 1.5.16 and zlib 1.2.8
node-spritesheet checked out from master today.
latest node and grunt from NPM.

I've not looked at pngcrush options too much, maybe there's some extra options to keep alpha channel with reduce-option. Will check back later.