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/lzma/package.py

@@ -19,4 +19,4 @@ class Lzma(AutotoolsPackage):
     homepage = "http://tukaani.org/lzma/"
     url      = "http://tukaani.org/lzma/lzma-4.32.7.tar.gz"
 
-    version('4.32.7', '2a748b77a2f8c3cbc322dbd0b4c9d06a')
+    version('4.32.7', sha256='9f337a8c51e5ded198d1032f5087ba3fe438f2a54e9df419e513a151775b032c')