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/boostmplcartesianproduct/package.py

@@ -15,7 +15,7 @@ class Boostmplcartesianproduct(Package):
     homepage = "http://www.organicvectory.com/index.php?option=com_content&view=article&id=75:boostmplcartesianproduct&catid=42:boost&Itemid=78"
     url      = "https://github.com/quinoacomputing/BoostMPLCartesianProduct/tarball/20161205"
 
-    version('20161205', 'b0c8534ee807484ffd161723cbc8fc04')
+    version('20161205', sha256='1fa8e367e4dc545b34016bf57d802858ce38baf40aff20f7c93b329895a18572')
 
     def install(self, spec, prefix):
         install_tree('boost/mpl', join_path(prefix.include, 'boost', 'mpl'))