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.
Removing overlap with tox.cover config in tox coveralls.
Fixing comment description in update_docs.sh.
Merge pull request #301 from dhermes/speed-up-build-stage-one

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