travis: test on node 8 and 10
travis: test on node 8 and 10

I think now it's pretty much the norm to test on the two main node.js branches like this, which are currently 8.x and 10.x.
mlarghydracept/p5.js,mlarghydracept/p5.js,bomoko/p5.js,limzykenneth/p5.js,joecridge/p5.js,dhowe/p5.js,bomoko/p5.js,processing/p5.js,dhowe/p5.js,joecridge/p5.js,limzykenneth/p5.js,processing/p5.js