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/r-proto/package.py

@@ -15,4 +15,4 @@ class RProto(RPackage):
     list_url = "https://cloud.r-project.org/src/contrib/Archive/proto"
 
     version('1.0.0', sha256='9294d9a3b2b680bb6fac17000bfc97453d77c87ef68cfd609b4c4eb6d11d04d1')
-    version('0.3-10', 'd5523943a5be6ca2f0ab557c900f8212')
+    version('0.3-10', sha256='d0d941bfbf247879b3510c8ef3e35853b1fbe83ff3ce952e93d3f8244afcbb0e')