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/neuron/package.py

@@ -22,10 +22,10 @@ class Neuron(Package):
     git      = "https://github.com/nrnhines/nrn.git"
 
     version('develop', branch='master')
-    version('7.5', 'fb72c841374dfacbb6c2168ff57bfae9')
-    version('7.4', '2c0bbee8a9e55d60fa26336f4ab7acbf')
-    version('7.3', '993e539cb8bf102ca52e9fefd644ab61')
-    version('7.2', '5486709b6366add932e3a6d141c4f7ad')
+    version('7.5', sha256='67642216a969fdc844da1bd56643edeed5e9f9ab8c2a3049dcbcbcccba29c336')
+    version('7.4', sha256='1403ba16b2b329d2376f4bf007d96e6bf2992fa850f137f1068ad5b22b432de6')
+    version('7.3', sha256='71cff5962966c5cd5d685d90569598a17b4b579d342126b31e2d431128cc8832')
+    version('7.2', sha256='c777d73a58ff17a073e8ea25f140cb603b8b5f0df3c361388af7175e44d85b0e')
 
     variant('mpi',           default=True,  description='Enable MPI parallelism')
     variant('python',        default=True,  description='Enable python')