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-alabaster/package.py

@@ -16,7 +16,7 @@ class PyAlabaster(PythonPackage):
     import_modules = ['alabaster']
 
     version('0.7.12', sha256='a661d72d58e6ea8a57f7a86e37d86716863ee5e92788398526d58b26a4e4dc02')
-    version('0.7.10', '7934dccf38801faa105f6e7b4784f493')
-    version('0.7.9',  'b29646a8bbe7aa52830375b7d17b5d7a')
+    version('0.7.10', sha256='37cdcb9e9954ed60912ebc1ca12a9d12178c26637abdf124e3cde2341c257fe0')
+    version('0.7.9',  sha256='47afd43b08a4ecaa45e3496e139a193ce364571e7e10c6a87ca1a4c57eb7ea08')
 
     depends_on('py-setuptools', type='build')