Add Travis CI config

[mbrukman]

I have enabled this repo on Travis CI.

What we need now is a PR with a small .travis.yml file that will build and test the code, before we start making any cleanup or other changes here.

You can see what is currently done for the main JanusGraph repo, but we don't need anything close to that level of complexity, we can have something much smaller and simpler to start with here.

[farodin91]

I think would better to start switching to the travis-ci.com.

[mbrukman]

FWIW, I went to https://travis-ci.com/github/JanusGraph/janusgraph-foundationdb first, but even after signing in, it says:

We couldn't display the repository JanusGraph/janusgraph-foundationdb

This repository may not exist or you may need to use another account to have access to it

Sign in with another account

which is why I then went to https://travis-ci.org/github/JanusGraph/janusgraph-foundationdb and activated it there.

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It looks like we have to do a migration from travis-ci.org to travis-ci.com; if I visit https://travis-ci.com/organizations/JanusGraph/repositories, I see the following:

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While it might be a good idea to start using travis-ci.com, I think it's a much bigger change, and we will end up migrating the entire org at the same time, and we will also switch from using the webhook to using Travis as a GitHub App, and that's bound to introduce new changes and potential complications.

I would propose we decouple the addition of a .travis.yml to this repo (which is blocking cleanup and other enhancement work) from migrating to travis-ci.com and also the migration to the Travis-as-GitHub-App. We should probably do the test with the main repo first (github.com/JanusGraph/janusgraph) to ensure that it works there, and if it does, it should work for all other smaller, less complex repos as well.

Thoughts?