D-BUS daemon outputs a lot of warnings in travis tests

Description

During travis job execution of the functional tests a lot of unneccesary logs/output are produced, when running selenium tests.

Relevant versions used in our functional tests:

• Selenium version 3.141.0
• Geckodriver version 0.23.0
• Firefox browser version 61.0.1

An example of the logs:

`(firefox:6677): GConf-WARNING **: Client failed to connect to the D-BUS daemon: Unable to autolaunch a dbus-daemon without a $DISPLAY for X11

(firefox:6677): GConf-WARNING **: Client failed to connect to the D-BUS daemon: Unable to autolaunch a dbus-daemon without a $DISPLAY for X11

(firefox:6677): GLib-GObject-WARNING **: invalid (NULL) pointer instance

(firefox:6677): GLib-GObject-CRITICAL **: g_signal_connect_data: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(firefox:6677): GLib-GObject-CRITICAL **: g_object_get: assertion 'G_IS_OBJECT (object)' failed

(firefox:6677): GLib-GObject-CRITICAL **: g_object_get: assertion 'G_IS_OBJECT (object)' failed

(firefox:6677): GConf-WARNING **: Client failed to connect to the D-BUS daemon: Unable to autolaunch a dbus-daemon without a $DISPLAY for X11

(firefox:6677): GLib-GObject-CRITICAL **: g_object_get: assertion 'G_IS_OBJECT (object)' failed

(firefox:6677): GLib-GObject-CRITICAL **: g_object_get: assertion 'G_IS_OBJECT (object)' failed

(firefox:6677): GConf-WARNING **: Client failed to connect to the D-BUS daemon: Unable to autolaunch a dbus-daemon without a $DISPLAY for X11

(firefox:6677): GConf-WARNING **: Client failed to connect to the D-BUS daemon: Unable to autolaunch a dbus-daemon without a $DISPLAY for X11`

Motivation

Reducing these rows makes it easier to skim through the logs of the functional tests, if something goes wrong. This output does not affect test result but makes it harder to find cause of test errors, if they occur.

Benefits

Cleaner logs during test execution in travis.

Possible Drawbacks

None.