utils/external_packages.py: increase maximum package size

Signed-off-by: Cleber Rosa <crosa@redhat.com>
gwt: update to version 2.5.1

This task is described on issue #606.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
frontent/client: simple fix to make it work with gwt 2.5.1

Signed-off-by: Cleber Rosa <crosa@redhat.com>