build: fix Travis non-PR builds
Don't return non-zero if $TRAVIS_PULL_REQUEST == "false".
Refs: https://github.com/nodejs/node/pull/24076#issuecomment-435716318
Checklist
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commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Don't return non-zero if $TRAVIS_PULL_REQUEST == "false".
Refs: https://github.com/nodejs/node/pull/24076#issuecomment-435716318