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/perl-www-robotrules/package.py

@@ -12,6 +12,6 @@ class PerlWwwRobotrules(PerlPackage):
     homepage = "http://deps.cpantesters.org/?module=WWW%3A%3ARobotRules;perl=latest"
     url      = "http://search.cpan.org/CPAN/authors/id/G/GA/GAAS/WWW-RobotRules-6.02.tar.gz"
 
-    version('6.02', 'b7186e8b8b3701e70c22abf430742403')
+    version('6.02', sha256='46b502e7a288d559429891eeb5d979461dd3ecc6a5c491ead85d165b6e03a51e')
 
     depends_on('perl-uri', type=('build', 'run'))