Explicitly testing dependency versions supported on travis

[machow]

Some key questions:

• Should we support pandas~=0.23.0? Likely not too hard, requires patching (1) ser.shift which is missing the fill_value argument, (2) no ser.dt.timetz property, (3) what appears to be a small bug in ser.pct_change?

• Should we test pip install ./ and tests on all supported python versions? (3.6, 3.7, 3.8)

• What about testing mac, windows, etc..?

I've got this 90% of the way on https://github.com/machow/siuba/tree/travis-py38

[machow]

Supporting v0.23.0 is probably not worth it right now. This is unfortunate, since v0.24.0 was released Jan 2019, so we depend on a version released a year ago.

(But this is probably the least of siuba's problems)