Revert version-specific CF standard-names usage in .travis.yml.

Just reverts https://github.com/cpelley/iris/commit/b4dafb5538ee1f9b3c3d88bd5e1cc6b0c110b7d6,
which was included in https://github.com/SciTools/iris/pull/586

We suspect that the published CF "latest" version has now been fixed 
 -- Since my PR https://github.com/SciTools/iris/pull/589 yesterday checked out ok, it seems that version 24, published 2013-06-28, may have been "quietly" changed since, without assigning a new revision number

So, if **this** PR checks out, it should be okay to merge.