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/cask/package.py

@@ -18,9 +18,9 @@ class Cask(Package):
     homepage = "http://cask.readthedocs.io/en/latest/"
     url      = "https://github.com/cask/cask/archive/v0.7.4.tar.gz"
 
-    version('0.8.1', '25196468a7ce634cfff14733678be6ba')
+    version('0.8.1', sha256='8739ba608f23c79b3426faa8b068d5d1bc096c7305ce30b1163babd354be821c')
     # version 0.8.0 is broken
-    version('0.7.4', 'c973a7db43bc980dd83759a5864a1260')
+    version('0.7.4', sha256='b183ea1c50fc215c9040f402b758ecc335901fbc2c3afd4a7302386c888d437d')
 
     depends_on('emacs', type=('build', 'run'))