make test assert conditional on mapnik version - finishes #560
make test assert conditional on mapnik version - finishes #560

tomhughes/node-mapnik,langateam/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,langateam/node-mapnik,langateam/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,CartoDB/node-mapnik,tomhughes/node-mapnik,CartoDB/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik