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.

.travis.yml

@@ -42,3 +42,5 @@ branches:
   only:
   - master
   - "/^v\\d+\\.\\d+\\.\\d+$/"
+  - "/^v\\d+\\.\\d+\\.\\d+-beta.\d+$/"
+  - "/^v\\d+\\.\\d+\\.\\d+-alpha.\d+$/"