Removing skip travis from commit msg so wheelhouse rebuilds occur.

Also removing an out-of-date comment.
Updating CONTRIBUTING to describe build optimizations.
Speeding up lint by only considering files which are different.

H/T to @sk- and https://github.com/sk-/git-lint for the
inspiration.
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.
storage: update api version
Merge pull request #317 from silvolu/fix-storage-api-version

storage: update api version
Merge pull request #316 from dhermes/speed-up-lint

Speeding up lint by only considering files which are different.
Fix Copyright example in LICENSE
Merge pull request #309 from dhermes/wheels-tweak-and-docs

Wheels tweak and docs
Refactoring duplicate code between storage.key and storage.bucket.