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

@@            Coverage Diff            @@
##             master     #127   +/-   ##
=========================================
  Coverage     31.72%   31.72%           
  Complexity       60       60           
=========================================
  Files             6        6           
  Lines           249      249           
=========================================
  Hits             79       79           
  Misses          170      170           

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