Print a warning when configured mapnik version doesn't match installed

lib/cartodb/server_options.js

@@ -21,6 +21,14 @@ module.exports = function(){
         log_format: global.environment.log_format
     };
 
+    // Be nice and warn if configured mapnik version
+    // is != instaled mapnik version
+    if ( mapnik.versions.mapnik != me.grainstore.mapnik_version ) {
+      console.warn("WARNING: detected mapnik version ("
+        + mapnik.versions.mapnik + ") != configured mapnik version ("
+        + me.grainstore.mapnik_version + ")");
+    }
+
     // Set the cache chanel info to invalidate the cache on the frontend server
     //
     // @param req The request object.