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/py-monotonic/package.py

@@ -12,6 +12,6 @@ class PyMonotonic(PythonPackage):
     homepage = "https://pypi.python.org/pypi/monotonic"
     url      = "https://pypi.io/packages/source/m/monotonic/monotonic-1.2.tar.gz"
 
-    version('1.2', 'd14c93aabc3d6af25ef086b032b123cf')
+    version('1.2', sha256='c0e1ceca563ca6bb30b0fb047ee1002503ae6ad3585fc9c6af37a8f77ec274ba')
 
     depends_on('py-setuptools', type='build')