Force compatible EventDispatcher version in integration tests

Nextcloud requires EventDispatcher from Symfony 4.4. Behat required
Symfony 4.x until Behat 3.5, but since Behat 3.6 it supports Symfony 5.x
too. However, as the EventDispatcher version was not restricted in the
"composer.json" file Composer installed the latest compatible version
with all the dependencies, which happened to be Symfony 5.x. To prevent
that now the EventDispatcher is explicitly limited to Symfony 4.4 only.

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