add compatibility with mapnik 2.3.x
add compatibility with mapnik 2.3.x

tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,tomhughes/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile,mapbox/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,landsurveyorsunited/mapnik-vector-tile