Travis: Build also node 12
Travis: Build also node 12

tomhughes/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik