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/cvs/package.py

@@ -12,6 +12,6 @@ class Cvs(AutotoolsPackage):
     homepage = "http://www.nongnu.org/cvs/"
     url      = "https://ftpmirror.gnu.org/non-gnu/cvs/source/feature/1.12.13/cvs-1.12.13.tar.bz2"
 
-    version('1.12.13', '93a8dacc6ff0e723a130835713235863f1f5ada9')
+    version('1.12.13', sha256='78853613b9a6873a30e1cc2417f738c330e75f887afdaf7b3d0800cb19ca515e')
 
     parallel = False