Switch Slack image to a working tag

We've had reports of the `:latest` tag not working anymore,
leading to an error as described here:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341

roles/matrix-bridge-appservice-slack/defaults/main.yml

@@ -3,7 +3,7 @@
 
 matrix_appservice_slack_enabled: true
 
-matrix_appservice_slack_docker_image: "cadair/matrix-appservice-slack:latest"
+matrix_appservice_slack_docker_image: "cadair/matrix-appservice-slack:cadair"
 matrix_appservice_slack_docker_image_force_pull: "{{ matrix_appservice_slack_docker_image.endswith(':latest') }}"
 
 matrix_appservice_slack_base_path: "{{ matrix_base_data_path }}/appservice-slack"