set up .travis.yml

Good evening!

It has been quite some time since I contributed to cli-table, but I am participating in 24pullrequests, so I figured I would take a look and see if any additional work was needed.

I figured it couldn't hurt to have the repository on TravisCI (and I recall some tests being broken when I made some changes last year), so I went ahead and set that up.  You can see a passing build here:  https://travis-ci.org/sonnym/cli-table/builds/14782592

I hope this helps out a bit!