README.md: requirements info (PHP 7.1 vs. 7.4)

[k00ni]

README.md used another minimum PHP version (7.1) than composer.json does (7.4). I assume composer.json's version is the correct one.

[malberts]

The whole Requirements section is outdated. PHP, MW and SMW versions need to be updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.40%. Comparing base (067b121) to head (7de7d77).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #94   +/-   ##
=========================================
  Coverage     83.40%   83.40%           
  Complexity       80       80           
=========================================
  Files             7        7           
  Lines           223      223           
=========================================
  Hits            186      186           
  Misses           37       37           

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