Revert "Update riot-web (0.17.6 -> 0.17.7)"

As Github issue #42 says, 0.17.7 is not available.
We shouldn't have tried to use it.

This reverts commit 529b5b222d.

roles/matrix-server/defaults/main.yml

@@ -262,7 +262,7 @@ matrix_corporal_debug: false
 # you may wish to disable this.
 matrix_riot_web_enabled: true
 
-matrix_riot_web_docker_image: "avhost/docker-matrix-riot:v0.17.7"
+matrix_riot_web_docker_image: "avhost/docker-matrix-riot:v0.17.6"
 
 matrix_riot_web_data_path: "{{ matrix_base_data_path }}/riot-web"