update .travis.yml and fix numpy-pandas dependency issue

[peterstangl]

This PR updates the .travis.yml file (removes Python 3.5 and adds Python 3.8 and 3.9) and fixes a dependency issue between pandas and numpy by requiring numpy >= 1.16.5.

[coveralls]

Pull Request Test Coverage Report for Build 214

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.899%

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Totals
Change from base Build 210:	0.0%
Covered Lines:	5634
Relevant Lines:	6267

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💛 - Coveralls

[DavidMStraub]

Strange, why would Travis install such an old numpy version (and only for 3.7, not for 3.6)?

[peterstangl]

Strange, why would Travis install such an old numpy version (and only for 3.7, not for 3.6)?

Yes, I also don't understand why Travis installs this numpy version with Python 3.7. In principle, this is not an issue of wilson and I'm not sure if it should be addressed in the setup.py. But on the other hand, it's very easy to fix and I think it doesn't hurt to require numpy >= 1.16.5.