Bump to Node v6?

[pdehaan]

Currently we're using Node 5.x in the .travis.yml file, but not sure if we want to bump up to 6.x (since it's the latest stable) or if we want to stick with Node 5 for consistency with activity-stream.

[rittme]

We want it to be consistent with activity-stream, since the plan is to merge this back.
I would say we should keep this version to avoid issues when migrating.