Bump acorn from 6.2.1 to 6.4.1

Bumps [acorn](https://github.com/acornjs/acorn) from 6.2.1 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/compare/6.2.1...6.4.1)

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

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "operator-mono-lig",
-  "version": "2.2.7",
+  "version": "2.2.8",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -379,9 +379,9 @@
       "dev": true
     },
     "acorn": {
-      "version": "6.2.1",
-      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.2.1.tgz",
-      "integrity": "sha512-JD0xT5FCRDNyjDda3Lrg/IxFscp9q4tiYtxE1/nOzlKCk7hIRuYjhq1kCNkbPjMRMZuFq20HNQn1I9k8Oj0E+Q==",
+      "version": "6.4.1",
+      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.4.1.tgz",
+      "integrity": "sha512-ZVA9k326Nwrj3Cj9jlh3wGFutC2ZornPNARZwsNYqQYgN0EsV2d53w5RN/co65Ohn4sUAUtb1rSUAOD6XN9idA==",
       "dev": true
     },
     "ajv": {