Travis: add libcurl for curlInterface; don't build Digraphs

[olexandr-konovalov]

The new curlInterface requires libcurl4-openssl-dev. For the 32-bit build one should add libcurl4-openssl-dev:i386.

[codecov]

Codecov Report

Merging #2768 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2768      +/-   ##
==========================================
+ Coverage   75.81%   75.81%   +<.01%     
==========================================
  Files         481      481              
  Lines      241517   241517              
==========================================
+ Hits       183105   183106       +1     
+ Misses      58412    58411       -1

Impacted Files	Coverage Δ
hpcgap/lib/hpc/stdtasks.g	64.19% <0%> (+0.25%)	⬆️

[olexandr-konovalov]

Also, in 64-bit build, the test fail when it comes to loading digraphs: an undefined symbol: atexit error occur:

+bin/gap.sh --quitonbreak -q -L testpackagesload.wsp
Loading digraphs ... 
#W dlopen() error: /home/travis/build/gap-system/gap/pkg/digraphs-0.12.2/bin/x\
86_64-pc-linux-gnu-default64/digraphs.so: undefined symbol: atexit
Error, module '/home/travis/build/gap-system/gap/pkg/digraphs-0.12.2/bin/x86_64-pc-li\
nux-gnu-default64/digraphs.so' not found in
  if not LOAD_DYN( arg[1], false ) then
    Error( "no support for dynamic loading" );
fi; at /home/travis/build/gap-system/gap/lib/files.gd:583 called from 

[olexandr-konovalov]

@james-d-mitchell I have temporary disabled Digraphs build here, to avoid undefined symbol: atexit problem. We did the same with the Float package. We have seen this before for Digraphs - see digraphs/Digraphs#107. But Digraphs builds fine in GAP Docker container in both branches, and has perfectly runnable tests. I don't think there is a regression in Digraphs or/and core GAP system - seems that it also depends on some luck with getting the right Travis environment, which is outside our control.

P.S. And we already tried to revert that hack, and it worked for a while, until the problem reappeared: #2391

[fingolfin]

While I still hope to fix the issues with digraphs, float and semigroups (e.g. via PR #2759), let's be pragmatic and fix the broken build now and then work on a "better" fix afterwards.