bump beta version

3.18.1-beta.2

Last commit before the attempt to integrate rust via neon-bindings

3.18.1-beta.1

Have Travis build Beta and Alpha releases.

The last beta build didn't actually trigger the expected build
because the branch (well, tag) wasn't white-listed.
This should fix that problem.

3.18.0

3.17.7

3.17.6

3.17.5

3.17.4