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/loki/package.py

@@ -13,7 +13,7 @@ class Loki(MakefilePackage):
     homepage = "http://loki-lib.sourceforge.net"
     url      = "https://downloads.sourceforge.net/project/loki-lib/Loki/Loki%200.1.7/loki-0.1.7.tar.bz2"
 
-    version('0.1.7', '33a24bcbb99fa2ec8fcbbab65649f3f6')
+    version('0.1.7', sha256='07553754f6be2738559947db69b0718512665bf4a34015fa3a875b6eb1111198')
 
     variant('shared', default=True, description="Build shared libraries")