tracer: update to newest version of CLS

In retrospect, I probably should have bumped CLS to 4.0.0, but the bits that changed in `node-newrelic` were using undocumented internals that I originally piggybacked upon to avoid needing 2 async listeners to handle CLS *and* error tracing. As a workaround, I updated the dependency in `package.json` so that you won't somehow end up with a version of CLS with the old internal API.

`continuation-local-storage@3.1.0` includes @groundwater's fix for othiym23/async-listener#19 / othiym23/async-listener#20, which is the main motivator for the upgrade. All tests are passing (although you should probably upgrade `CONTRIBUTING.md` to reflect the fact that `boot2docker` is also a dependency for running the integration tests).