Bump version of pip to reflect change in docker image
python:3.6-slim upgraded its version of pip: https://github.com/docker-library/python/commit/bbb44b3ab29fb1d60dbe59c5a7ea4af868657ff2#diff-233602ef1ec9c3947dd4b5e114fdf756
python:3.6-slim upgraded its version of pip: https://github.com/docker-library/python/commit/bbb44b3ab29fb1d60dbe59c5a7ea4af868657ff2#diff-233602ef1ec9c3947dd4b5e114fdf756