Uses pre-built wheels when pip installing from tox.

In addition:
- Creates script scripts/update_wheels_project.sh to kick off a new
  build when merging to master (outside of a PR).
- Moves update_docs into a larger group of merge scripts
  that happen when merging to master.

Fixes #290 

Relies on https://github.com/GoogleCloudPlatform/gcloud-python-wheels/pull/2 being merged and the build successfully completing (to create the wheelhouse).