tests(travis): test on Node 9, drop testing on Node 7 (#4181)
tests(travis): test on Node 9, drop testing on Node 7 (#4181)


mixed/lighthouse,GoogleChrome/lighthouse,deepanjanroy/lighthouse,GoogleChrome/lighthouse,ev1stensberg/lighthouse,ev1stensberg/lighthouse,GoogleChrome/lighthouse,umaar/lighthouse,ev1stensberg/lighthouse,umaar/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,mixed/lighthouse,GoogleChrome/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse,mixed/lighthouse,GoogleChrome/lighthouse,mixed/lighthouse,umaar/lighthouse,wardpeet/lighthouse,tkadlec/lighthouse,umaar/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,tkadlec/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse,deepanjanroy/lighthouse,deepanjanroy/lighthouse,ev1stensberg/lighthouse