Switch CI to use GitHub Actions

[fingolfin]

This is necessary as Travis CI started to require payment for its services

[codecov[bot]]

Codecov Report

Merging #36 (9ccf33a) into master (411e1ad) will increase coverage by 0.80%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   90.10%   90.90%   +0.80%     
==========================================
  Files          22       22              
  Lines        2364     2463      +99     
==========================================
+ Hits         2130     2239     +109     
+ Misses        234      224      -10     

Impacted Files	Coverage Δ
gap/place.gd	100.00% <0.00%> (ø)
gap/presentation.gd	100.00% <0.00%> (ø)
gap/small_pregroups.gi	99.20% <0.00%> (+<0.01%)	:arrow_up:
gap/anadata.gi	98.63% <0.00%> (+0.03%)	:arrow_up:
gap/pregroupconstr.gi	95.83% <0.00%> (+0.05%)	:arrow_up:
gap/walrus.gi	95.57% <0.00%> (+0.09%)	:arrow_up:
gap/place.gi	93.28% <0.00%> (+0.26%)	:arrow_up:
gap/util.gi	92.00% <0.00%> (+0.33%)	:arrow_up:
gap/presentation.gi	94.17% <0.00%> (+0.90%)	:arrow_up:
gap/pregroup.gi	82.73% <0.00%> (+1.39%)	:arrow_up:
... and 4 more