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/gtkorvo-enet/package.py

@@ -16,5 +16,5 @@ class GtkorvoEnet(AutotoolsPackage):
     homepage = "http://www.github.com/GTkorvo/enet"
     url = "https://github.com/GTkorvo/enet/archive/v1.3.13.tar.gz"
 
-    version('1.3.14', '05272cac1a8cb0500995eeca310e7fac')
-    version('1.3.13', '3490f924a4d421e4832e45850e6ec142')
+    version('1.3.14', sha256='d1fda051bdee46ad8cce7c3bb36fb6b7a7a443945f27a280ac104753c29465b0')
+    version('1.3.13', sha256='ede6e4f03e4cb0c3d93044ace9e8c1818ef4d3ced4aaa70384155769b3c436dc')