Require GAP 4.15

[fingolfin]

That's the first version of GAP that shipped with utils >= 0.81 and groupoids >= 1.78.

As such this should fix the minimal CI job. The fix for the broken devel job is in PR #164.

An alternative fix would be to keep the GAP bound, and instead install the latest utils and groupoids versions before starting the CI tests. But then we are no longer testing "can one just drop this package into an older GAP and it will work". But that's of course your choice :-)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (4181f08) to head (95d0ff7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   79.54%   79.56%   +0.02%     
==========================================
  Files          38       38              
  Lines       14335    14335              
==========================================
+ Hits        11403    11406       +3     
+ Misses       2932     2929       -3     

see 1 file with indirect coverage changes

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[fingolfin]

Rebased this to verify that CI is now fully fixed with this patch