Update dependencies to get ready for node v6

src/serve_rendered.js

@@ -16,7 +16,7 @@ var Canvas = require('canvas'),
     express = require('express'),
     mercator = new (require('@mapbox/sphericalmercator'))(),
     mbgl = require('@mapbox/mapbox-gl-native'),
-    mbtiles = require('mbtiles'),
+    mbtiles = require('@mapbox/mbtiles'),
     pngquant = require('node-pngquant-native'),
     proj4 = require('proj4'),
     request = require('request');