Travis Configuration and Build Optimizations

All build scripts in the ``.travis.yml`` configuration file which have

Python dependencies are specified in the ``tox.ini`` configuration.

They are executed in the Travis build via ``tox -e {ENV}`` where

``{ENV}`` is the environment being tested.

By enumerating all Python dependencies in the ``tox`` configuration,

we can use our custom ``gcloud-python-wheels``

`wheelhouse <https://github.com/GoogleCloudPlatform/gcloud-python-wheels>`__

to speed up builds. This project builds and stores pre-built Python

`wheels <http://pythonwheels.com>`__ for every Python dependency our library

and tests have.

If new ``tox`` environments are added to be run in a Travis build, they

should either be:

- listed in ``[tox].envlist`` as a default environment

- added to the list in the

`Travis environment variable <http://docs.travis-ci.com/user/environment-variables/#Using-Settings>`__

``EXTRA_TOX_ENVS``. The current value is::