ci: skip `yarn install` on Travis (#3278)
ci: skip `yarn install` on Travis (#3278)

Summary:
This was previously needed to run `yarn lint`, but we run that on GitHub
Actions now (in parallel with Travis!), so it’s entirely superfluous and
takes about a minute to run.

Test Plan:
Ensure that Travis is still happy.

wchargin-branch: ci-travis-no-yarn

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard