Docs: update gulp recipe to return promise chain. Fixes #1955 (#1956)
Docs: update gulp recipe to return promise chain. Fixes #1955 (#1956)

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