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 #311 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #311   +/-   ##
=========================================
  Coverage     67.88%   67.88%           
  Complexity       23       23           
=========================================
  Files             4        4           
  Lines           109      109           
=========================================
  Hits             74       74           
  Misses           35       35           

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