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/compiz/package.py

@@ -17,7 +17,7 @@ class Compiz(AutotoolsPackage):
     homepage = "http://www.compiz.org/"
     url      = "https://www.x.org/archive/individual/app/compiz-0.7.8.tar.gz"
 
-    version('0.7.8', 'e99977d9170a7bd5d571004eed038428')
+    version('0.7.8', sha256='b46f52b776cc78e85357a07688d04b36ec19c65eadeaf6f6cfcca7b8515e6503')
 
     depends_on('libxcb')
     depends_on('libxcomposite')