Add support for Travis CI

This changeset adds support for [Travis CI](https://travis-ci.org/) as publicly visible means of continuous integration (additionally to the probably internally running CI system at Google).

Travis CI is configured to build Guava with Oracle JDK 7, OpenJDK 7, and OpenJDK 6. Oracle JDK 8 hasn't been added to the list because of the currently unresolved issues with building Guava with Java 8.

In order for this changeset to work, someone with the appropriate permissions on the Guava repository will have to activate the build on Travis CI, see [Activate GitHub Webhook](http://docs.travis-ci.com/user/getting-started/#Step-two%3A-Activate-GitHub-Webhook).