travis: enable DTLS

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10720 ddefafc4-47db-0310-ae44-fa13212b10f2
resip/stack: DTLS support for certificate chains

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10721 ddefafc4-47db-0310-ae44-fa13212b10f2