Upgrade riot-web (1.5.6 -> 1.5.7)

roles/matrix-riot-web/defaults/main.yml

@@ -1,6 +1,6 @@
 matrix_riot_web_enabled: true
 
-matrix_riot_web_docker_image: "vectorim/riot-web:v1.5.6"
+matrix_riot_web_docker_image: "vectorim/riot-web:v1.5.7"
 matrix_riot_web_docker_image_force_pull: "{{ matrix_riot_web_docker_image.endswith(':latest') }}"
 
 matrix_riot_web_data_path: "{{ matrix_base_data_path }}/riot-web"