Added travis CI integration

[Srokap]

• [ ] Setup AWS S3 buckets for testing.
• [ ] Need to encode AWS_KEY and AWS_SECRET env vars using travis encrypt for the original repo, see https://docs.travis-ci.com/user/encryption-keys/
• [x] Need to point all badges to the main organization
• [ ] Need to enable builds on TravisCI for main organization and set concurrency to one job at a time (in Travis UI settings)

See https://travis-ci.org/Srokap/knox for the tests running on my fork. See https://coveralls.io/github/Srokap/knox for how the coveralls integration looks like.

For testing I chose oldest branch I was able to run tests on (the 0.8 defined in package.json was failing on npm install), LTS and current version which is 6. For now we run them in series, so didn't want to add more than I felt was necessary. If we created separate buckets for them, they could run in parallel.

[Srokap]

Tomorrow passes one year since I created this PR and it's obvious that there's no interest in it. Closing.