Add more variants for Travis builds

[olexandr-konovalov]

Started to work on #55.

This now checks all combinations of GMP (builtin/system) x ABI (32/64) x Build(standard or out-of-tree), except system GMP in 32-bit builds (because "trusty" is missing 32bit GMP, see travis-ci/apt-package-safelist#4018).

TODOs (for this or rather another PR):

• add testing builds in HPC-GAP compatibility mode
• add AppVeyor builds

[codecov-io]

Codecov Report

Merging #61 into hpc-merge will decrease coverage by 2.44%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           hpc-merge      #61      +/-   ##
=============================================
- Coverage       56.8%   54.35%   -2.45%     
=============================================
  Files            433      859     +426     
  Lines         231015   244891   +13876     
  Branches       10695    11895    +1200     
=============================================
+ Hits          131228   133117    +1889     
- Misses         97284   108638   +11354     
- Partials        2503     3136     +633

Impacted Files	Coverage Δ
src/intrprtr.c	58.83% <0%> (-7.6%)	❌
src/objects.h	62.5% <0%> (-5.93%)	❌
src/rational.c	85.46% <0%> (-3.01%)	❌
src/streams.c	37.17% <0%> (-2.98%)	❌
src/trans.c	92.66% <0%> (-2.95%)	❌
src/gvars.c	72.52% <0%> (-2.84%)	❌
src/set.c	81.44% <0%> (-2.62%)	❌
src/weakptr.c	77.6% <0%> (-2.61%)	❌
src/opers.c	55.02% <0%> (-2.27%)	❌
src/profile.c	33.33% <0%> (-2.16%)	❌
... and 482 more

---

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[fingolfin]

I just pushed some changes to the branch which should help compiling io. You may want to rebase your PR onto that.