Fix: Replace npm with yarn on travis (#2067)
Fix: Replace npm with yarn on travis (#2067)

includes yarn caching
cedricbellet/lighthouse,GoogleChrome/lighthouse,wardpeet/lighthouse,deepanjanroy/lighthouse,wardpeet/lighthouse,cedricbellet/lighthouse,mixed/lighthouse,arturmiz/lighthouse,arturmiz/lighthouse,GoogleChrome/lighthouse,cedricbellet/lighthouse,tkadlec/lighthouse,GoogleChrome/lighthouse,deepanjanroy/lighthouse,deepanjanroy/lighthouse,ev1stensberg/lighthouse,mixed/lighthouse,umaar/lighthouse,deepanjanroy/lighthouse,tkadlec/lighthouse,ev1stensberg/lighthouse,mixed/lighthouse,deepanjanroy/lighthouse,GoogleChrome/lighthouse,umaar/lighthouse,ev1stensberg/lighthouse,wardpeet/lighthouse,mixed/lighthouse,umaar/lighthouse,cedricbellet/lighthouse,arturmiz/lighthouse,wardpeet/lighthouse,umaar/lighthouse,ev1stensberg/lighthouse,arturmiz/lighthouse,tkadlec/lighthouse,wardpeet/lighthouse,arturmiz/lighthouse,tkadlec/lighthouse,cedricbellet/lighthouse,GoogleChrome/lighthouse