Test against Node 0.11

This makes the same changes to Socket.IO's .travis.yml in PR https://github.com/LearnBoost/socket.io/pull/1425.
It tests against Node 0.8 and 0.11 on Travis CI, and allows the Node 0.11 job to fail without failing the whole build. The `fast_finish: true` line allows the build to be marked as finished without waiting for the Node 0.11 job to finish.