Added Node.js 7 to package.json. This way yarn will not complain when
Node.js 7 is used. Tests still pass and the code itself also works
on Node.js 7.
Configure Travis CI to run the build on Node.js 7 too, in order to
verify correctness.
Rework the module to use the new cls-hooked module (which uses AsyncWrap
available since Node v4.5) instead of old continuation-local-storage
(based on very unreliable async-listener).