build: move lint & prettier to a standalone Travis-CI build step

[bajtos]

See #601 (comment)

Just a comment regarding checking this in our npm test workflow. I think it might be better to have different stage for linting on travis where we run npm run lint && npm run prettier:check. I just find it easier to look at Travis and know where something went wrong without having to open up logs.

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[dhmlau]

@bajtos, is this something we still want to do?

[raymondfeng]

We already have a code-lint step in travis.