travis/coverity: add coverity support

Run only on push to branch coverity-scan. We can use a cron script to do
this 4 times a week (that's our allowance).

NOTE: possible future improvements are:

1. Fold the build matrix item into another short one so we don't overburden
   travis. It's a little less clear but it should be nicer on the
   infrastructure.
2. Change the security token, one can do that from the coverity admin page.
3. Don't do the naive `make depend`, but use the prebuilt libraries.
readme: add coverity scan badge