Forced rebuild because of merge issues with astropy_helpers

* The commit history where astropy_helpers accidentally got
  modified is making rebasing off of master really messy, so
  rebuilt from current.
* CHANGES.rst not modified, so this is *not* ready to go.
  Should wait on 0.4 to shake out before updating that.
Update CHANGES to include cosmology clone operation

* Placed in 1.0 section
* Closes #2592
Merge pull request #2592 from aconley/copy_cosmo

Making slightly different cosmologies is awkward