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/appres/package.py

@@ -15,7 +15,7 @@ class Appres(AutotoolsPackage):
     homepage = "http://cgit.freedesktop.org/xorg/app/appres"
     url      = "https://www.x.org/archive/individual/app/appres-1.0.4.tar.gz"
 
-    version('1.0.4', 'f82aabe6bbb8960781b63c6945bb361b')
+    version('1.0.4', sha256='22cb6f639c891ffdbb5371bc50a88278185789eae6907d05e9e0bd1086a80803')
 
     depends_on('libx11')
     depends_on('libxt')