Use consistent function declaration style

src/engine/variable.js

@@ -9,8 +9,10 @@
  * @param {boolean} isCloud Whether the variable is stored in the cloud.
  * @constructor
  */
-module.exports = function Variable (name, value, isCloud) {
+var Variable = function (name, value, isCloud) {
     this.name = name;
     this.value = value;
     this.isCloud = isCloud;
 };
+
+module.exports = Variable;