Fix coverage job

[nlohmann]

• make sure gcov and gcc use the same version
• replace deprecated parameter lcov_branch_coverage with branch_coverage
• suppressed errors

The fix reduces the coverage, but the uncovered lines are all false positives, like the second line of a multi-line return statement where the first line is marked as covered.

[coveralls]

coverage: 99.184% (-0.5%) from 99.639%
when pulling fe00b36 on fix-coverage
into 2d42229 on develop.