Introducing custom pip install in tox to be used with cached wheels.

In addition:
- Removes used of "pip install" in Travis config with our custom
  pip install.
- Introducing tox environment "coveralls" to encourage all Python
  dependencies to be contained in "tox.ini".
- Puts LICENSE header back on update_docs.sh and cleaned up some
  bash long line / syntax with shellchecker.

See https://github.com/GoogleCloudPlatform/gcloud-python-wheels/pull/1#issuecomment-60648806
step 2 for context.

-----

This is towards a fix of #290 and the comment above describes the full path.

@silvolu After this is merged, I will restart the failed wheelhouse build
https://travis-ci.org/GoogleCloudPlatform/gcloud-python-wheels/builds/39208014
and the error

> `gcloud-python does not contain a custom pip install script.`

will be resolved.
You can DRY this via [tox config](http://tox.readthedocs.org/en/latest/config.html#substitution-for-values-from-other-sections), e.g.:

    [testenv:coveralls]
    basepython = {[cover]basepython}
    commands =
        {[cover]commands}
        coveralls
    deps =
        {[cover]deps}
        coveralls