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/imake/package.py

@@ -12,7 +12,7 @@ class Imake(AutotoolsPackage):
     homepage = "http://www.snake.net/software/imake-stuff/"
     url      = "https://www.x.org/archive/individual/util/imake-1.0.7.tar.gz"
 
-    version('1.0.7', '186ca7b8ff0de8752f2a2d0426542363')
+    version('1.0.7', sha256='6bda266a07eb33445d513f1e3c82a61e4822ccb94d420643d58e1be5f881e5cb')
 
     depends_on('xproto', type='build')
     depends_on('pkgconfig', type='build')