Disable travis e2e tests for now

Connection with Sauce Labs seems unreliable, looks like the session is
closed mid-test-run sometimes, but could be something else.

Maybe re-using the same webdriver instance is a bad idea (but makes the
tests run faster so can keep doing it locally), and we should use a new
one for each test suite, to "refresh" the connection.

Maybe the selenium tests themselves are the problem.

Until things are more reliable, only run unit tests on Travis, and run
e2e tests locally.
Update to platform-client 0.4.1