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/platypus/package.py

@@ -12,7 +12,7 @@ class Platypus(Package):
     homepage = "http://www.well.ox.ac.uk/platypus"
     url      = "http://www.well.ox.ac.uk/bioinformatics/Software/Platypus-latest.tgz"
 
-    version('0.8.1', 'edf3fb5bf080241ddb75a413c8529d57')
+    version('0.8.1', sha256='a0f39e800ebdc5590e9b568a791bc6746df0fde4d1c3622140db64dea175622b')
 
     depends_on('python@2.6:', type=('build', 'run'))
     depends_on('py-cython', type='build')