Bump acorn from 6.1.0 to 6.4.1

Bumps [acorn](https://github.com/acornjs/acorn) from 6.1.0 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.1.0...6.4.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -53,7 +53,7 @@
     "darwin"
   ],
   "devDependencies": {
-    "acorn": "^6.0.4",
+    "acorn": "^6.4.1",
     "acorn-jsx": "^5.0.0",
     "chai": "^4.2.0",
     "eslint": "^5.9.0",