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-graph-readwrite/package.py

@@ -12,4 +12,4 @@ class PerlGraphReadwrite(PerlPackage):
     homepage = "http://search.cpan.org/~neilb/Graph-ReadWrite/lib/Graph/Writer/Dot.pm"
     url      = "http://search.cpan.org/CPAN/authors/id/N/NE/NEILB/Graph-ReadWrite-2.09.tar.gz"
 
-    version('2.09', '5cd9191eadd2fe8fe8bb431575434f67')
+    version('2.09', sha256='b01ef06ce922eea12d5ce614d63ddc5f3ee7ad0d05f9577051d3f87a89799a4a')