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Also removing an out-of-date comment.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
Normalize away extra empty line after method docstring.
Merge remote-tracking branch 'upstream/master'
Moar empty-lines-after-docstring.
Remove 'poetry' in docstrings.
Remove moar spurious blank lines, docstring poetry.
Even moar spurious blank lines, docstring poetry.
Moar spurious blank lines.
Make 'Bucket.delete_keys' return a useful result.

Result is a list of (key, success) tuples, one per requested key.

Fixes #161.

Addresses #160 (users who want to delete without raising can use
'Bucket.delete_keys([key])' instead of 'Bucket.delete_key(key)').
Merge branch '161-bucket-delete_keys-useful_result' of github.com:tseaver/gcloud-python into 161-bucket-delete_keys-useful_result