Let ghostscript keep it's own delegates or it will complain about png incompatibilities

vagrant@vagrant-ubunutu-12-04:/usr/src/imagemagick-static$ target/bin/compare test/output/lolcat.pdf ./test/fixtures/lolcat.pdf
libpng warning: Application was compiled with png.h from libpng-1.2.42
libpng warning: Application  is  running with png.c from libpng-1.4.4
libpng warning: Incompatible libpng version in application and library