dynamically choose mapnik-reference version based on mapnik version
use mapnik version rather than node-mapnik version
grab correct mapnik-ref for validation
Merge branch 'mb-pages' into dynamic-mapnik-ref

Conflicts:
	lib/server.js
Merge pull request #890 from mapbox/dynamic-mapnik-ref

[NOT READY] dynamically choose mapnik-reference version based on mapnik version