Update to node-mapnik 1.3.x - refs mapnik/node-mapnik#209

API changes needed are minor

TODO:

  - [x] - ensure tests pass on ubuntu - https://travis-ci.org/mapbox/tilelive-vector/builds/18915659
  - [x] - Failing image comparison tests on OS X - seems like the tests are not working correctly in a way that is unrelated to the new node-mapnik version
  - [x] - restart travis tests once a non-alpha node-mapnik tag is ready

Seeing on OS X:

```
 1) tiles should render b (1.0.0):
     TypeError: Cannot read property '1' of null
      at ChildProcess.<anonymous> (/Users/dane/projects/tilelive-vector/test/image.js:40:85)
      at ChildProcess.EventEmitter.emit (events.js:98:17)
      at Process.ChildProcess._handle.onexit (child_process.js:797:12)
```