Add travis ci support

This adds travis CI support.  

A couple tests are broken: https://travis-ci.org/taketime/tilelive-vector/jobs/10547433   

[This test](https://github.com/taketime/tilelive-vector/blob/master/test/test.js#L81) is triggering a ['No Source' error](https://github.com/taketime/tilelive-vector/blob/master/index.js#L68) rather than a 'No Backend' error, as the test doesn't set a `uri.source`.  Is this testing what you mean it to?  What's a source and what's a backend?

Fixes #8