CI test to make sure bokeh works without heavy dependencies

[mattpap]

There should be a separate test that makes sure that bokeh works without ~~~numpy~~~, scipy, pandas, etc. The minimal environment should contain python (obviously), six and preferably nothing more. Make sure to test not only if bokeh imports like this, but test as many plotting APIs as feasible

removed comments about charts

[bryevdv]

I think I am inclined to just expand this test (or make a new one just like it):

https://github.com/bokeh/bokeh/blob/master/tests/test_no_tornado_common.py

to include pandas scipy and call it a day. I stopped trying to run the unit tests after having to install boto, colorama, requests, and PIL, and it's not clear to me that trying to run API code is that valuable. As a general rule we import at the top of modules so this kind of test should go a long way.

[bryevdv]

Well that doesn't quite work since we do conditionally import pandas if it is present in some places. We would need an actual different environment. I'm not sure it's worth it, we have not needed this so far and I think a pandas or scipy import would be noticed in review.

Other idea though, we could add a NO_PD_SP flag to the py3.5 unit test run, and have that rest explicity uninstall pandas and scipy before running unit tests