DOC/CI: restore travis CI doc build environment
xref https://github.com/pandas-dev/pandas/pull/26591#issuecomment-498040997

This is one option (adding back the specific doc build environment yml file), another option would be to actually update the main `environment.yaml` file to be suitable for the doc build. 
At the moment, my preference is to add back the separate yml file because 1) those additional dependencies are not necessarily needed for a dev env setup, and just makes that env even more heavy and 2) we might want to pin to certain versions from time to time for the doc build (eg we should consider pinning sphinx to < 2.0, which is not necessarily needed for all contributors to do as well).

Another thing is that this env could actually use an update (eg python 3.6 -> 3.7)

cc @datapythonista 
