Reduce async.queue concurrency to 3

Due to a process being spawned for every pipeline run, concurrency should remain around 3 to keep number of child processes from running away and allowing the CPU to stay on top.

lib/core/engine.js

@@ -170,7 +170,7 @@ class Engine {
     const cargo = async.queue((task, callback) => {
       generateCode(task.modifiedAsset);
       self.events.once('outputDone', callback);
-    }, 10);
+    }, 3);
     const addToCargo = function (modifiedAsset) {
       cargo.push({modifiedAsset});
     };