README.md: fix Travis/Codecov/Coveralls URL

[fingolfin]

I also enabled coveralls for this repo, fixed the codecov integration, and removed the outdated travis.org "GitHub Service" integration (leaving the travis.com "GitHub Apps" integration in place -- now Travis won't have to build every PR and commit twice ;-).

[codecov]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #70   +/-   ##
=======================================
  Coverage   46.33%   46.33%           
=======================================
  Files          30       30           
  Lines        2398     2398           
=======================================
  Hits         1111     1111           
  Misses       1287     1287

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