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

@@            Coverage Diff            @@
##             master     #289   +/-   ##
=========================================
  Coverage     73.52%   73.52%           
  Complexity      230      230           
=========================================
  Files            25       25           
  Lines           982      982           
=========================================
  Hits            722      722           
  Misses          260      260

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