Use the proper freeing method

As according to
http://www.postgresql.org/docs/9.0/static/libpq-exec.html#LIBPQ-EXEC-ESCAPE-STRING

Signed-off-by: Dan Farina <daniel@heroku.com>

dbutils.c

@@ -210,7 +210,7 @@ getMasterConnection(PGconn *standby_conn, int id, char *cluster,
 											  strlen(schema_str));
 
 		maxlen_snprintf(schema_quoted, "%s", identifier);
-		free(identifier);
+		PQfreemem(identifier);
 	}
 
 	/* find all nodes belonging to this cluster */