travis: Stop passing --coverage.
travis: Stop passing --coverage.

This causes a bunch of noise in the CI logs, which we don't
really look at.  It'd make more sense if we were close to
100% test coverage in Jest, so that exceptions stood out.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile