Releasing

1. Make changes.
2. Determine next version. For the sake of this list, let's assume 1.2.3.
3. Update gax.go with const Version = "1.2.3". If v2, do the same to v2/gax.go.
4. git checkout -b my_branch && git commit && git push my_remote my_branch
5. Get it reviewed and merged.
6. git checkout master && git tag v1.2.3 && git push origin v1.2.3
7. Update the releases page on GitHub