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-rematch/package.py

@@ -15,4 +15,4 @@ class RRematch(RPackage):
     url      = "https://cloud.r-project.org/src/contrib/rematch_1.0.1.tar.gz"
     list_url = "https://cloud.r-project.org/src/contrib/Archive/rematch"
 
-    version('1.0.1', '5271666295e232931f21499522489dd3')
+    version('1.0.1', sha256='a409dec978cd02914cdddfedc974d9b45bd2975a124d8870d52cfd7d37d47578')