CI: Simplify and unify environment creation
When creating a pandas environment for the CI, we currently have two separate systems, one for Travis and one for Azure. Summarizing, those are the involved steps.

**travis**
```
ci/prep_cython_cache.sh
ci/install_travis.sh         -> locale, download/install conda, ccache, create env, setup.py develop
ci/submit_cython_cache.sh
ci/install_db_travis.sh      -> create dbs
ci/before_script_travis.sh   -> xvfb (clipboard)
```

**azure**
```
ci/incremental/install_miniconda.sh          -> download/install conda
ci/incremental/setup_conda_environment.sh    -> create env, locale
ci/incremental/build.sh                      -> setup.py build_ext
```

Except for the cython cache, I think all the rest could be unified in a single file `ci/setup_env.sh` that performs all the steps.

This would simplify and avoid duplicate code and inconsistencies. As well as making future changes simpler.