🏗🚮 Simplify the way gulp lint works on Travis

[rsimha]

Over the past year, various lint rules were added to improve AMP's code correctness. For example, #14742, #12748, #15255, #14761. In addition, lint checks were enabled for the code in validator/, which is developed in a different repo and periodically synced to amphtml. See #18601 and #13966.

As a result, gulp lint is run only against the files edited by a PR during PR builds, while hundreds of linter warnings are printed during push builds.

This PR does the following:

• Silences 200+ outstanding linter warnings for amphtml-internal/prefer-deferred-promise, amphtml-internal/unused-private-field, and amphtml-internal/vsync, and for the validator/ directory
• Updates lint.js to lint all files during Travis PR and push builds
• Paves the way for 🏗✨ Adopt prettier for code formatting  #21212 by starting with a clean slate of linter errors

Follow up tasks:

• Fix remaining amphtml-internal/prefer-deferred-promise, amphtml-internal/unused-private-field, and amphtml-internal/vsync errors and re-enable the rules in error mode (Find unused (dead) class fields #14742, Layers: Move element mutation logic into Layers #12748)
• Fix lint errors in validator/ and re-enable all rules in validator/.eslintrc (Fix all remaining lint warnings in validator/ #18601, Include validator code in lint and presubmit checks  #13966)

[honeybadgerdontcare]

lgtm validator

[rsimha]

This is ready. Logs now show zero lint errors / warnings: https://travis-ci.org/ampproject/amphtml/jobs/532494203#L346-L350

I've also tested that running gulp lint --local-changes still works as originally designed.

[estherkim]

lint task change LGTM