Require GAP >= 4.12

[fingolfin]

... as this package now uses Last() which was introduced then.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.70%. Comparing base (1b98b9c) to head (cc5625d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #106   +/-   ##
=======================================
  Coverage   59.70%   59.70%           
=======================================
  Files          78       78           
  Lines       23901    23901           
=======================================
  Hits        14271    14271           
  Misses       9630     9630           

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