webUI acceptance test Mink-selenium dependencies

[phil-davis]

overnight the UI acceptance tests started failing, e.g. see test PR #37111
https://drone.owncloud.com/owncloud/core/23882/86/15

  Scenario: simple user login                                                                 # /drone/src/tests/acceptance/features/webUILogin/login.feature:12
    Given these users have been created with default attributes and without skeleton files:   # FeatureContext::theseUsersHaveBeenCreatedWithDefaultAttributesAndWithoutSkeletonFiles()
      | username |
      | user1    |
    When user "user1" logs in using the webUI                                                 # WebUILoginContext::logInWithUsernameUsingTheWebUI()
      Fatal error: Call to a member function open() on null (Behat\Testwork\Call\Exception\FatalThrowableError)
    Then the user should be redirected to a webUI page with the title "Files - %productname%" # WebUIGeneralContext::theUserShouldBeRedirectedToAWebUIPageWithTheTitle()
SCENARIO RESULT: (fail)

Seems to be related to the new releases of behat/mink-selenium2-driver and behat/mink that were done overnight.

https://github.com/minkphp/MinkSelenium2Driver/releases/tag/v1.4.0
https://github.com/minkphp/Mink/releases/tag/v1.8.1

• pin the version of whatever dependencies to get tests passing again [Tests-Only] Pin behat/mink to version 1.7.1 #37112
• bump dependencies to whatever newer/well-defined versions will work reliably
• investigate what "official" versions have trouble, and why, and either fix our code or report upstream...