Switch Matrix Docker images (silviof -> AVENTER-UG)

Silvio announced that he's no longer maintaining his images,
so we're jumping to AVENTER-UG's fork.

README.md

@@ -51,9 +51,9 @@ Special thanks goes to:
 
 - [EMnify/matrix-synapse-auto-deploy](https://github.com/EMnify/matrix-synapse-auto-deploy) - for the inspiration
 
-- [silviof/docker-matrix](https://hub.docker.com/r/silviof/docker-matrix/) - for packaging Matrix Synapse as a Docker image
+- [silviof/docker-matrix](https://hub.docker.com/r/silviof/docker-matrix/) - for packaging Matrix Synapse as a Docker image. Note: due to maintenance issues, we're now using [AVENTER-UG/docker-matrix](https://github.com/AVENTER-UG/docker-matrix)'s fork.
 
-- [silviof/matrix-riot-docker](https://hub.docker.com/r/silviof/matrix-riot-docker/) - for packaging Riot as a Docker image
+- [silviof/matrix-riot-docker](https://hub.docker.com/r/silviof/matrix-riot-docker/) - for packaging Riot as a Docker image. Note: due to maintenance issues, we're now using [AVENTER-UG/docker-matrix-riot](https://github.com/AVENTER-UG/docker-matrix-riot)'s fork.
 
 
 ## Prerequisites

roles/matrix-server/defaults/main.yml

@@ -38,9 +38,9 @@ matrix_nginx_riot_web_data_path: "{{ matrix_base_data_path }}/riot-web"
 matrix_scratchpad_dir: "{{ matrix_base_data_path }}/scratchpad"
 
 docker_postgres_image: "postgres:9.6.6-alpine"
-docker_matrix_image: "silviof/docker-matrix:latest"
+docker_matrix_image: "avhost/docker-matrix:v0.26.0"
-docker_nginx_image: "nginx:1.13.7-alpine"
+docker_nginx_image: "nginx:1.13.8-alpine"
-docker_riot_image: "silviof/matrix-riot-docker:latest"
+docker_riot_image: "avhost/docker-matrix-riot:latest"
 docker_s3fs_image: "xueshanf/s3fs:latest"
 
 # To avoid Synapse's macaroon secret key from changing every time