update to node-mapnik@1.3.x
use node-mapnik@1.3.0-alpha2
bump to next node-mapnik@1.3.0-alpha3
bump to node-mapnik@1.3.0-alpha4
use node-mapnik@1.3.0
Merge pull request #107 from mapbox/node-mapnik-1.3

Update to node-mapnik@1.3.x