Fix PostgreSQL URI

SQLAlchemy removed support for `postgres://`, the URI has to start with `postgresql://`.

modoboa_installer/scripts/automx.py

@@ -34,7 +34,7 @@ class Automx(base.Installer):
         """Additional variables."""
         context = super(Automx, self).get_template_context()
         sql_dsn = "{}://{}:{}@{}/{}".format(
-            self.dbengine,
+            "postgresql" if self.dbengine == "postgres" else self.dbengine,
             self.config.get("modoboa", "dbuser"),
             self.config.get("modoboa", "dbpassword"),
             self.dbhost,