Update riot-web (0.15.5 -> 0.15.6)

roles/matrix-server/defaults/main.yml

@@ -65,7 +65,7 @@ docker_postgres_image_latest: "{{ docker_postgres_image_v10 }}"
 
 docker_matrix_image: "matrixdotorg/synapse:v0.31.2"
 docker_nginx_image: "nginx:1.15.0-alpine"
-docker_riot_image: "avhost/docker-matrix-riot:v0.15.5"
+docker_riot_image: "avhost/docker-matrix-riot:v0.15.6"
 docker_s3fs_image: "xueshanf/s3fs:latest"
 docker_goofys_image: "cloudproto/goofys:latest"
 docker_coturn_image: "instrumentisto/coturn:4.5.0.7"