Add slack notifications [skip ci]
Add slack notifications [skip ci]

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