Duplicated some Travis CI builds with --enable-hpcgap option

[olexandr-konovalov]

Addresses part of #55

[codecov-io]

Codecov Report

Merging #81 into hpc-merge will decrease coverage by 0.18%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           hpc-merge      #81      +/-   ##
=============================================
- Coverage      64.39%   64.21%   -0.19%     
=============================================
  Files            860      860              
  Lines         245691   245011     -680     
  Branches       11918    11893      -25     
=============================================
- Hits          158220   157322     -898     
+ Misses         84707    84338     -369     
- Partials        2764     3351     +587

Impacted Files	Coverage Δ
src/intrprtr.c	61.17% <0%> (-7.6%)	❌
src/objects.h	62.5% <0%> (-5.93%)	❌
lib/csetperm.gi	87.65% <0%> (-3.71%)	❌
src/rational.c	88.08% <0%> (-2.99%)	❌
src/trans.c	93.13% <0%> (-2.94%)	❌
src/streams.c	42.3% <0%> (-2.9%)	❌
src/gvars.c	76.33% <0%> (-2.82%)	❌
src/weakptr.c	84.89% <0%> (-2.61%)	❌
src/opers.c	57.5% <0%> (-2.48%)	❌
src/set.c	81.7% <0%> (-2.37%)	❌
... and 67 more

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