update pngcrush-installer for v0.6.14

* updates grunticon version from 0.6.14 to 0.6.15
* fixes tarball download and build by updating pngcrush-installer https://github.com/jlembeck/pngcrush-installer/pull/12

I think you don't want to merge this into master since you have beta in master. But I don't know how to send the patch since there is no v0.6.14 branch.