Overhaul Travis & Codecov setup

[fingolfin]

For Codecov, actually enable status messages in pull requests (so far,
all were turned off, but for some reason codecov ignores this; still,
better to specify actual intent in the file).

For Travis, adapt scripts/build_pkg.sh to work with packages not using
autoconf, so that we can use the same script across many packages. Also
don't copy the tested package into GAP's pkg dir; instead, put it into
its own GAP root which we tell GAP to look at before its usual GAP root.

Modify scripts/gather-coverage.sh and scripts/run_tests.sh to use this
modified list of GAP root dirs

Simplify scripts/build_gap.sh significantly by using BuildPackages.sh.

[codecov]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   35.62%   35.62%           
=======================================
  Files           2        2           
  Lines         379      379           
  Branches       67       69    +2     
=======================================
  Hits          135      135           
  Misses        222      222           
  Partials       22       22

[fingolfin]

Hmm, it seems the Travis builds are failing on master, so of course they are failing for this PR, too. The test suite is somehow segfaulting, hum.

Failures started when Use C99 init for StructInitInfo was merged, but the tests for that PR did work. That seems fishy.

Perhaps the problem is caused by Travis having updated some software, e.g. perhaps their zeromq is now a different version? Should be possible to tell by inspecting and comparing the travis build logs.

[fingolfin]

I fixed the build failures on the master branch, now tests pass here, too.