Slack appservice: Enable RTM by default

It is very confusing to debug why messages only go from Matrix to Slack
but not from Slack to Matrix. RTM should be enabled by default, as
that's the recommended way to make this work.

roles/matrix-bridge-appservice-slack/templates/config.yaml.j2

@@ -9,6 +9,9 @@ homeserver:
     url: "{{ matrix_appservice_slack_homeserver_url }}"
     media_url: "{{ matrix_appservice_slack_homeserver_media_url }}"
 
+rtm:
+    enable: true
+
 {% if matrix_appservice_slack_database_engine == 'nedb' %}
 dbdir: "/data"
 {% else %}