Pin behat/mink to 1.7.1 and update other acceptance test dependencies

[phil-davis]

From core issue owncloud/core#37113

behat/mink has to be pinned to 1.7.1 for now.

And while we are here, update the other acceptance test dependencies so they "document" the current minor releases that actually get used.

Note: symfony/translation 4.4 requires at least PHP 7.1.3 - that is why I changed platform php to 7.1.3

[codecov]

Codecov Report

Merging #163 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #163   +/-   ##
=========================================
  Coverage     67.75%   67.75%           
  Complexity       66       66           
=========================================
  Files            14       14           
  Lines           276      276           
=========================================
  Hits            187      187           
  Misses           89       89

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