chore(ci): drop node 4 and add node 10 + npm 6
chore(ci): drop node 4 and add node 10 + npm 6

BREAKING CHANGE: This library will no longer be tested Node,js 4 and as such will not be guaranteed to work on this version of Node.js. This library supports LTS versions of Node.js as they are specified here: https://github.com/nodejs/Release#release-schedule

closes #47

smartcar/clutch-assert