disable Travis CI job using libpcap-dev

The recent PPPoE session ID test case in commit 1ed68a5 depends on the
latest libpcap version to pass. The tcpdump build will fail until the
libpcap feature makes it into the next libpcap release and eventually
into the build server's libpcap-dev package, which won't happen soon.

For the time being leave only the job using git master of libpcap to
build tcpdump, such that Travis status on the pull requests is accurate.