Coverity scan fails

[mourner]

@mrgreywater you've added Coverity Scan for this repo many years ago, and now the badge in the readme indicates a scan failure — looks like the last successful scan was 2 years ago, and it's been failing since with this error:

Error details: The build uploaded has not been fully compiled. Please fix all compilation errors for accurate analysis.

Is that scan still relevant? Should we disable it and remove the badge, or address the compiling issue to fix it?

[mrgreywater]

Sometimes it shows passing (that's what it currently shows for me), sometimes it does not. I could spend some time troubleshooting why it started failing in the build process, but I think I'll just remove the badge and the integration in the travis build altogether. I feel it has outlived it's utility.

[kkaefer]

Add coverage again in #128