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/netgauge/package.py

@@ -15,6 +15,6 @@ class Netgauge(AutotoolsPackage):
     homepage = "http://unixer.de/research/netgauge/"
     url      = "http://unixer.de/research/netgauge/netgauge-2.4.6.tar.gz"
 
-    version('2.4.6', 'e0e040ec6452e93ca21ccc54deac1d7f')
+    version('2.4.6', sha256='dc9398e4e042efec70881f2c7074ff18cc5b74bc5ffc4b8a4aaf813b39f83444')
 
     depends_on("mpi")