fix: webpack 3 RuleSet creation (#407)

Author: mzgoddard

lib/util/serial.js

@@ -74,11 +74,20 @@ const loaders = (exports.loaders = {
           ruleSet = extra.compiler.__hardSource_ruleSet;
           if (!ruleSet) {
             const RuleSet = require('webpack/lib/RuleSet');
-            ruleSet = extra.compiler.__hardSource_ruleSet = new RuleSet(
-              extra.compiler.options.module.defaultRules.concat(
-                extra.compiler.options.module.rules,
-              ),
-            );
+            if (extra.compiler.options.module.defaultRules) {
+              // webpack 4
+              ruleSet = extra.compiler.__hardSource_ruleSet = new RuleSet(
+                extra.compiler.options.module.defaultRules.concat(
+                  extra.compiler.options.module.rules,
+                ),
+              );
+            } else {
+              // webpack <4
+              ruleSet = extra.compiler.__hardSource_ruleSet = new RuleSet(
+                extra.compiler.options.module.rules ||
+                  extra.compiler.options.module.loaders,
+              );
+            }
           }
         }
         return {