Coverage reporting broken for newer versions of gcc(gcov)

[hendrikmuhs]

After bumping the ubuntu image to 20.04 (to fix rust builds) the coverage report broke and e.g. reported wrong line counts and as a result of that wrong coverage. The image for creating coverage reports has been set back to 16.04.

The problem turned out to be a problem with the extended format of gcov introduced in gcc 8, it reports coverage for instances of templates. The parser from cpp-coveralls produces wrong results as result of that, I created an upstream fix: eddyxu/cpp-coveralls#157.

[narekgharibyan]

Hah, great find @hendrikmuhs!

Hope they will accept the PR soon, as the repo didn't seem to be actively maintained.