checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/ghostscript-fonts/package.py

@@ -13,7 +13,7 @@ class GhostscriptFonts(Package):
     homepage = "http://ghostscript.com/"
     url = "https://www.imagemagick.org/download/delegates/ghostscript-fonts-std-8.11.tar.gz"
 
-    version('8.11', '6865682b095f8c4500c54b285ff05ef6')
+    version('8.11', sha256='0eb6f356119f2e49b2563210852e17f57f9dcc5755f350a69a46a0d641a0c401')
 
     def install(self, spec, prefix):
         fdir = join_path(prefix.share, 'font')