walkthrough setup.md tf version and geopandas installation error and fix.

While trying to setup training env, I came across the following error -

- python3.6 does not support tf v1.*
- geopandas had some broken file link to gdal (https://stackoverflow.com/questions/54734667/error-installing-geopandas-a-gdal-api-version-must-be-specified-in-anaconda).

The following two steps helped me go through these errors -

- use it to initiate an env with python 2.7 version
`virtualenv -p python2.7 venv`
- install the following two packages while installing _Install System Level Libraries_
`apt-get install libgeos-dev`
`apt-get install libgdal-dev`

Note that for ohsome2label we still need python3.6 version. One can use different virenv for that.

Hope it helps!