Bail on error in Travis

I think it's more useful to see that tests failed sooner than to see all
errors from a test run, particularly since earlier failures often cause
meaningless later failures anyway. And since spurious failures are
fairly common, this will also let us restart tests more quickly without
searching for "[FAIL]" in the output while it's running.

This does mean that, if there's a legitimate test failure you
overlooked, it might be worth running tests locally after fixing it to
make sure there aren't other instances of the problem that can be fixed
at the same time.

.travis.yml

@@ -49,4 +49,4 @@ before_script:
       fi
     - unset AWS_SECRET_ACCESS_KEY
 script:
-    - test/runtests.sh -x firefox/firefox
+    - test/runtests.sh -x firefox/firefox -f