Upgrade acorn to ^4.0.3

Author: timwis

package.json

@@ -7,7 +7,7 @@
     "browser-unpack": "bin/cmd.js"
   },
   "dependencies": {
-    "acorn": "^2.1.0",
+    "acorn": "^4.0.3",
     "browser-pack": "^5.0.1",
     "concat-stream": "^1.5.0",
     "minimist": "^1.1.1"