Set default cable adapter to postgres (#2096)

sample.env

@@ -252,6 +252,9 @@ DB_NAME=greenlight_production
 DB_USERNAME=postgres
 DB_PASSWORD=password
 
+# Use postgresql to handle ActionCable connections by default
+CABLE_ADAPTER=postgresql
+
 # Specify the default registration to be used by Greenlight until an administrator sets the
 # registration method
 # Allowed values are: