Disable MqttOpenTest.testConnectRetrySub from running on Travis

MqttOpenTest uses a publicly available Mosquitto MQTT server/broker, therefore some of the tests may timeout if the service is temporarily unavailable. On top of that, timing variances on the Travis CI shared servers may make it more likely for tests to fail.

Tests can be disabled from running on Travis CI, thus we avoid spurious Travis failures when people make a pull request.