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/pbbam/package.py

@@ -15,7 +15,7 @@ class Pbbam(CMakePackage):
     homepage = "https://github.com/PacificBiosciences/pbbam"
     url      = "https://github.com/PacificBiosciences/pbbam/archive/0.18.0.tar.gz"
 
-    version('0.18.0', 'abbb687b1e7ea08916c26da828e11384')
+    version('0.18.0', sha256='45286e5f7deb7ff629e0643c8a416155915aec7b85d54c60b5cdc07f4d7b234a')
 
     depends_on('zlib')
     depends_on('boost@1.55.0:')