Fails by default when running tests in travis

[ForbesLindesay]

Because we enable watch by default, tests will fail if you enable any CI server.

We can fix this in the majority of popular CI systems by checking for process.env.CI='true' (see https://docs.travis-ci.com/user/environment-variables/#Default-Environment-Variables), and then disabling --watch.

[gaearon]

Sounds good, would you like to make a PR? I was planning to suggest that people who need this create test-ci that is just react-scripts test but this seems even better. Maybe we can even kill explicit --watch in the config completely then and just respect CI env?