Add ghostscript as a required package.

Pillow uses ghostscript to manipulate eps images. Without ghostscript
being installed an obscure "OSError: [Errno 2] No such file or directory"
is raised on attempts to manipulate eps images. Since Pillow is among our
default pip-installed packages we should ensure this utility is there
always as well.
Merge pull request #134 from caktus/add-ghostscript-package

Add ghostscript as a required package.