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/ceres-solver/package.py

@@ -17,7 +17,7 @@ class CeresSolver(CMakePackage):
     homepage = "http://ceres-solver.org"
     url      = "http://ceres-solver.org/ceres-solver-1.12.0.tar.gz"
 
-    version('1.12.0', '278a7b366881cc45e258da71464114d9')
+    version('1.12.0', sha256='745bfed55111e086954126b748eb9efe20e30be5b825c6dec3c525cf20afc895')
 
     depends_on('eigen@3:')
     depends_on('lapack')