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/k8/package.py

@@ -14,7 +14,7 @@ class K8(Package):
     homepage = "https://github.com/attractivechaos/k8"
     url      = "https://github.com/attractivechaos/k8/releases/download/v0.2.4/k8-0.2.4.tar.bz2"
 
-    version('0.2.4', 'edc5579ff18842a2a59aa92ce8bab8b4')
+    version('0.2.4', sha256='da8a99c7f1ce7f0cb23ff07ce10510e770686b906d5431442a5439743c0b3c47')
 
     depends_on('zlib', type='run')