Remove CloudBees Snapshot and Pull Requests Jobs

A proposal to stop using CloudBees for pull requests (and snapshots) as a step towards migrating to Travis.

Travis is already doing pull requests. For example:

- https://travis-ci.org/ReactiveX/RxJava
- https://travis-ci.org/ReactiveX/BuildInfrastructure
- https://travis-ci.org/ReactiveX/RxJavaReactiveStreams
- https://travis-ci.org/ReactiveX/RxAndroid

I have added the .travis.yml file to all RxJava projects.

I have no idea if the changes in this commit are correct, but wanted to use it as a proposal.