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/axel/package.py

@@ -12,7 +12,7 @@ class Axel(AutotoolsPackage):
     homepage = "https://github.com/axel-download-accelerator/axel"
     url      = "https://github.com/axel-download-accelerator/axel/archive/v2.16.1.tar.gz"
 
-    version('2.16.1', '3aaa7052458c66c1547bf1133609d0f2')
+    version('2.16.1', sha256='64529add74df3db828f704b42d4ec3fcdacb8142c84f051f9213637c337e706c')
 
     depends_on('m4', type='build')
     depends_on('autoconf', type='build')