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/ndiff/package.py

@@ -18,8 +18,8 @@ class Ndiff(Package):
     homepage = "http://ftp.math.utah.edu/pub/ndiff/"
     url      = "http://ftp.math.utah.edu/pub/ndiff/ndiff-2.00.tar.gz"
 
-    version('2.00', '885548b4dc26e72c5455bebb5ba6c16d')
-    version('1.00', 'f41ffe5d12f36cd36b6311acf46eccdc')
+    version('2.00', sha256='f2bbd9a2c8ada7f4161b5e76ac5ebf9a2862cab099933167fe604b88f000ec2c')
+    version('1.00', sha256='d4be3ab38e4b87da8d689fe47413e01a7bfdf8c8627bfb673aac37953a463a92')
 
     def install(self, spec, prefix):
         configure('--prefix=%s' % prefix)