TAJO-814: Set up Travis CI builds.

I've tested this patch in my github account. It works well. Note that the stdout log size in Travis CI cannot exceed 4 MB. So, I add some grep filter to mvn command in .travis.yml.

Also, you can see how it works at https://travis-ci.org/hyunsik/tajo/builds.

After this patch is committed, I'll ask Travis CI integration to ASF infra team.