Bump Acorn from v2.x to v3.x

includes a breaking change https://github.com/ternjs/acorn/blob/master/CHANGELOG.md#300-2016-02-10

Author: shinnn

package.json

@@ -24,7 +24,7 @@
     "test": "node test/run.js"
   },
   "dependencies": {
-    "acorn": "^2.0.1"
+    "acorn": "^3.0.4"
   },
   "devDependencies": {
     "chai": "^3.0.0",