Be explicit about the map output srid configuration

lib/cartodb/server_options.js

@@ -29,6 +29,10 @@ module.exports = function(){
         base_url: '/tiles/:table',
         base_url_notable: '/tiles',
         grainstore: {
+          map: {
+            // TODO: allow to specify in configuration
+            srid: 3857
+          },
           datasource: global.environment.postgres,
           cachedir: global.environment.millstone.cache_basedir,
           mapnik_version: global.environment.mapnik_version || mapnik.versions.mapnik,