Reinsert eslint dependency assertion to package.json

Signed-off-by: Jeremy Ho <jujaga@gmail.com>

Author: jujaga

app/frontend/package.json

@@ -54,6 +54,7 @@
     "@vue/test-utils": "^1.2.2",
     "axios-mock-adapter": "^1.20.0",
     "babel-eslint": "^10.1.0",
+    "eslint": "^6.8.0",
     "eslint-plugin-prettier": "^3.4.0",
     "eslint-plugin-vue": "^7.18.0",
     "eslint-plugin-vuetify": "^1.0.1",