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/ctffind/package.py

@@ -12,7 +12,7 @@ class Ctffind(AutotoolsPackage):
     homepage = "http://grigoriefflab.janelia.org/ctffind4"
     url      = "http://grigoriefflab.janelia.org/sites/default/files/ctffind-4.1.8.tar.gz"
 
-    version('4.1.8', '8ae9d9abe363141a3792981b5a2fae94')
+    version('4.1.8', sha256='bec43c0b8d32878c740d6284ef6d9d22718c80dc62270be18d1d44e8b84b2729')
 
     depends_on('wxwidgets')
     depends_on('fftw@3:')