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/cub/package.py

@@ -13,9 +13,9 @@ class Cub(Package):
     homepage = "https://nvlabs.github.com/cub"
     url      = "https://github.com/NVlabs/cub/archive/1.6.4.zip"
 
-    version('1.7.1', '028ac43922a4538596338ad5aef0f0c4')
-    version('1.6.4', '924fc12c0efb17264c3ad2d611ed1c51')
-    version('1.4.1', '74a36eb84e5b5f0bf54aa3df39f660b2')
+    version('1.7.1', sha256='50b8777b83093fdfdab429a61fccdbfbbb991b3bbc08385118e5ad58e8f62e1d')
+    version('1.6.4', md5='924fc12c0efb17264c3ad2d611ed1c51')
+    version('1.4.1', md5='74a36eb84e5b5f0bf54aa3df39f660b2')
 
     def install(self, spec, prefix):
         mkdirp(prefix.include)