Use recent node versions in travis builds
Created by: davemevans
As per https://github.com/nodejs/LTS/#lts-schedule:
- Node 0.12 is no longer supported after 31st Dec 2016
- Active LTS versions are 4 and 6.
Node 0.12 seems to be causing intermittent failures of our Travis builds for network-related reasons, which are not seen on the Node 4 job. Removing it should make our Travis badge more likely to remain green.