Mapbox-tilesets

- Beautified error messages

- Use Bionic dist instead of Xenial for Travis build

- Pin urllib3 version, see: https://github.com/urllib3/urllib3/issues/2168

- Added command `tilesets list-activity` that returns activity data for a user's tilesets

- Bug Fix: Fix setup script to have `geojson` package in setup.py install requirements

*Yanked due to missing `geojson` package in setup.py*

- Validates source in `upload-source` command using [geojson package](https://github.com/jazzband/geojson).
- Provides line number for an invalid feature that is detected with the geojson validator.