Force compatible dependency versions in acceptance tests

behat/mink 1.8 and behat/mink-selenium2-driver 1.4 introduced behaviour
changes that broke the acceptance tests. Until the tests are updated to
work with the newer versions the last known versions are forced.

Note that some acceptance tests still fail after enforcing the
compatible versions, although that is caused by changes in the Nextcloud
server itself.

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