add coverity/scan-build to travis

[minad]

trafficstars

Run only on develop. There is a usage limit on coverity for example.

[sjaeckel]

isn't there a way to do this half-automatic via a special branch name!? ... right there's https://scan.coverity.com/travis_ci ...

I only did it manually for now in ltc but if you want to add support for it here I'll be glad to merge it (as soon as I find again the time to work on non-work projects...)!