Update Mink from 1.7.1 to 1.8.1 in acceptance tests

Since version 1.8.0 of Mink "Mink::getSession()" no longer starts the
session automatically (see https://github.com/minkphp/Mink/pull/705

), so
it now needs to be explicitly started.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>