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-reprex/package.py

@@ -21,7 +21,7 @@ class RReprex(RPackage):
 
     version('0.3.0', sha256='203c2ae6343f6ff887e7a5a3f5d20bae465f6e8d9745c982479f5385f4effb6c')
     version('0.2.1', sha256='5d234ddfbcadc5a5194a58eb88973c51581e7e2e231f146974af8f42747b45f3')
-    version('0.1.1', 'fcd89995d7b35a2ddd4269973937bde3')
+    version('0.1.1', sha256='919ae93039b2d8fb8eace98da9376c031d734d9e75c237efb24d047f35b5ba4b')
 
     depends_on('r@3.0.2:', when='@:0.1.2', type=('build', 'run'))
     depends_on('r@3.1:', when='@0.2.0:', type=('build', 'run'))