Bump acorn from 6.3.0 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.3.0 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/compare/6.3.0...6.4.2)

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

package-lock.json

@@ -33,9 +33,9 @@
       }
     },
     "acorn": {
-      "version": "6.3.0",
-      "resolved": "https://registry.npm.taobao.org/acorn/download/acorn-6.3.0.tgz?cache=0&sync_timestamp=1569334654641&other_urls=https%3A%2F%2Fregistry.npm.taobao.org%2Facorn%2Fdownload%2Facorn-6.3.0.tgz",
-      "integrity": "sha1-AIdQkRn/pPwKAEHR6TpBfmjLhW4="
+      "version": "6.4.2",
+      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.4.2.tgz",
+      "integrity": "sha512-XtGIhXwF8YM8bJhGxG5kXgjkEuNGLTkoYqVE+KMR+aspr4KGYmKYg7yUe3KghyQ9yheNwLnjmzh/7+gfDBmHCQ=="
     },
     "align-text": {
       "version": "0.1.4",