Add Travis CI

As much as I enjoy being a human continuous integration system, perhaps we can automate it for better and quicker coverage?

By adding this one tiny YAML file (yes, it's sad to pollute the repository with an ugly external configuration file, but that's just how it works), we can have six VMs launched and running Redis unit tests after every push.  Why six VMs?  {gcc,clang}{make test,make test-sentinel} + {gcc,clang}{make REDIS_CFLAGS=-Werror} = 6 VMs.

For example output, see https://travis-ci.org/mattsta/redis/builds/21050931 — you can see we currently have one failing test because of the `gcc -Werror` fixed in antirez/redis#1617

Travis CI will run on each push to any branch.  Just sign up at https://travis-ci.org/, login with github credentials, then enable it for your Redis repository.  By default, it will also build and report on pull requests too.