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/tree/package.py

@@ -18,7 +18,7 @@ class Tree(Package):
     homepage = "http://mama.indstate.edu/users/ice/tree/"
     url      = "http://mama.indstate.edu/users/ice/tree/src/tree-1.7.0.tgz"
 
-    version('1.7.0', 'abe3e03e469c542d8e157cdd93f4d8a6')
+    version('1.7.0', sha256='6957c20e82561ac4231638996e74f4cfa4e6faabc5a2f511f0b4e3940e8f7b12')
 
     def install(self, spec, prefix):
         objs = [