update microbundle to fix sourcemaps

Author: developit

package.json

@@ -25,7 +25,7 @@
   "devDependencies": {
     "eslint": "^4.16.0",
     "eslint-config-developit": "^1.1.1",
-    "microbundle": "^0.4.0",
+    "microbundle": "^0.4.1",
     "workerize-loader": "^1.0.1"
   },
   "dependencies": {