Core: reenable Travis testing

This is an effort to get Travis to green on ``core``.

There are a number of FIXMEs in ``requirements`` and in ``.travis.yml`` which should be removed when the status changes e.g. versions of Python and dependencies on pyDes.

Most notable issues are:

1. Docs now build without "warnings as errors" since there are a number of doc errors which would get fixed in #3307 
2. Installation is disabled because of #3378 and #3379
3. Migration testing is not ported since there is no chance of that working before the ``2.6.0`` interim release allows migration from ``2.6.0`` to ``core``.