Merge pull request #20049 from nextcloud/force-compatible-dependency-versions-in-acceptance-tests

Force compatible dependency versions in acceptance tests

tests/acceptance/composer.json

 {
   "require-dev": {
     "behat/behat": "^3.0",
-    "behat/mink": "^1.5",
+    "behat/mink": "1.7.1",
     "behat/mink-extension": "*",
-    "behat/mink-selenium2-driver": "*",
+    "behat/mink-selenium2-driver": "1.3.1",
     "phpunit/phpunit": "~4.6"
   },
   "autoload": {