Update Travis CI and Codecov to move it closer to the default one for GAP packages

[olexandr-konovalov]

This PR:

• adds standard .codecov.yml
• updates .travis.yml to test the package under more branches from the GAP repository
• adds Travis CI and Codecov badges to the README

The only distinction of .travis.yml here from e.g. the one in https://github.com/gap-packages/example is that RepnDecomp also calls ./makedoc.sh to extract manual examples into a test files (while some other packages extract examples too, but keep them under version control).

[codecov[bot]]

Codecov Report

Merging #6 into master will decrease coverage by 3.3%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   48.59%   45.28%   -3.31%     
==========================================
  Files          29       25       -4     
  Lines        1455     1367      -88     
==========================================
- Hits          707      619      -88     
  Misses        748      748

Impacted Files	Coverage Δ
PackageInfo.g
tst/testall.g
init.g
read.g

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[olexandr-konovalov]

@kaashif this is ready to be merged. The slight decrease in code coverage is due to us ignoring these files by default:

ignore:
  - "PackageInfo.g"
  - "init.g"
  - "read.g"
  - "tst/*"     # ignore test harness code
  - "tst/**/*"  # ignore test harness code