RFE: Use lcov files in Travis CI

[drakenclimber]

See this comment in Issue #299 for more details.

Our current code coverage solution (cpp-coveralls) has minor differences from the code coverage results generated from an lcov file. Initial analysis shows that the lcov file is likely more correct.

Switching from cpp-coveralls to an lcov file will also be beneficial if/when we need to transition from Travis CI to another continuous integration platform.

[pcmoore]

I'm closing this issue as it isn't relevant anymore now that we've migrated to GH Actions.