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/apex/package.py

@@ -12,7 +12,7 @@ class Apex(CMakePackage):
     homepage = "http://github.com/khuck/xpress-apex"
     url      = "http://github.com/khuck/xpress-apex/archive/v0.1.tar.gz"
 
-    version('0.1', 'e224a0b9033e23a9697ce2a3c307a0a3')
+    version('0.1', sha256='bb0be37f8f8133fe492998515bcf66a4df452c28a995d317228fbed9b18e6a92')
 
     depends_on("binutils+libiberty")
     depends_on("boost@1.54:")