Add missing peer dependency acorn

Author: cgsimmons

package-lock.json

@@ -37,6 +37,7 @@
     "@types/lodash.get": "4.4.6",
     "@typescript-eslint/eslint-plugin": "2.0.0",
     "@typescript-eslint/parser": "2.0.0",
+    "acorn": "^6.3.0",
     "concurrently": "4.1.2",
     "eslint": "6.2.0",
     "eslint-config-prettier": "6.1.0",