travis: test only with Mapnik 3.x
travis: test only with Mapnik 3.x

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