pip-requires: gui_topology requires webob>=1.2

gui_topology imports webob.static.DirecoryApp
which was added since webob 1.2

Signed-off-by: Hisaharu Ishii <ishii.hisaharu@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>