Switch to using docker to build bro and run tests on Travis CI.
Branch "topic/dnthayer/travis-ci-docker" in the bro git repo
changes the Travis CI config to use docker.
A number of other unrelated improvements were also made in
1) Fixed the problem of jobs sometimes failing due to the Travis
10-minute no output timeout.
2) Improved the overall speed of the build by skipping unneeded
components and tweaking the value of the "-j" parameter.
3) Prevent pull request builds from always failing by skipping
the private tests (pull request builds don't have access to the
4) Added more error checks and better error messages in the
"travis-job" script to make it more clear when something
unexpected happens (such as failure to get external tests).
5) Added output messages to separate the various stages
of the build for improved readability of the output.