Removing skip travis from commit msg so wheelhouse rebuilds occur.

Also removing an out-of-date comment.
Updating CONTRIBUTING to describe build optimizations.
Removing current value of EXTRA_TOX_ENVs from CONTRIBUTING.

Also changed the Travis settings in gcloud-python-wheels to
make this value visible so people can continue to maintain it.
Merge pull request #309 from dhermes/wheels-tweak-and-docs

Wheels tweak and docs