dropping down `requests` dependency to 2.0.0

re https://github.com/goldsmith/Wikipedia/issues/55

* updated requirements.txt with min of 2.0.0
* updated .travis.yml to test on 2.0.0, 2.1.0, 2.2.0, 2.3.0

I *think* the `travis` file should be correct.  that's the recommended approach in the docs;  I don't have a travis-ci.org account to test though (I should!)