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/abyss/package.py

@@ -15,8 +15,8 @@ class Abyss(AutotoolsPackage):
     url      = "https://github.com/bcgsc/abyss/releases/download/1.5.2/abyss-1.5.2.tar.gz"
 
     version('2.1.4', sha256='2145a1727556104d6a14db06a9c06f47b96c31cc5ac595ae9c92224349bdbcfc')
-    version('2.0.2', '1623f55ad7f4586e80f6e74b1f27c798')
-    version('1.5.2', '10d6d72d1a915e618d41a5cbbcf2364c')
+    version('2.0.2', sha256='d87b76edeac3a6fb48f24a1d63f243d8278a324c9a5eb29027b640f7089422df')
+    version('1.5.2', sha256='8a52387f963afb7b63db4c9b81c053ed83956ea0a3981edcad554a895adf84b1')
 
     variant('maxk', values=int, default=0,
             description='''set the maximum k-mer length.