Rework ci

This MR made some changes to the CI.

• The tests now run under continuumio/anaconda (python 2) and continuumio/anaconda3 (python 3).
• Coverage is still generated from python 3.
• The python 2 tests run all files matching test/*tests.py.

There are tests not being run under python 3 which are now being run under python 2. If we no longer want these, we should remove them.

I think it's worth running the examples in this way as it gives us an immediate check if we've broken any with other changes.

@paul-lasky @moritz.huebner @gregory.ashton