[CI only] Use RelWithDebInfo in Travis to get the callstacks

[levb]

Travis sanitize builds seem to still produce unique failures that are not appearing in GHA jobs. However, some were compiled with Release making call stack interpretation difficult (like, https://app.travis-ci.com/github/nats-io/nats.c/jobs/624866438). Use RelWithDebInfo to get the call stacks with symbols (hope it works in that environment, will see)