Sygnal: rollback to v0.15.1, because v0.16.0's docker image is broken

roles/custom/matrix-sygnal/defaults/main.yml

@@ -22,7 +22,7 @@ matrix_sygnal_hostname: ''
 matrix_sygnal_path_prefix: /
 
 # renovate: datasource=docker depName=matrixdotorg/sygnal
-matrix_sygnal_version: v0.16.0
+matrix_sygnal_version: v0.15.1
 
 matrix_sygnal_base_path: "{{ matrix_base_data_path }}/sygnal"
 matrix_sygnal_config_path: "{{ matrix_sygnal_base_path }}/config"