message: Drop object spread Babel plugin
  Follows
  Context in review comment:
diff: "webextension-polyfill": "^0.3.1"
  },
  "devDependencies": {
  - "@babel/core": "^7.0.0",
  - "@babel/plugin-syntax-object-rest-spread": "^7.0.0",
  + "@babel/core": "^7.1.5",
  "@babel/plugin-transform-modules-commonjs": "^7.0.0",
  "@babel/plugin-transform-react-jsx": "^7.0.0",
  "@babel/register": "^7.0.0",
  "pragma": "h",
  "useBuiltIns": true
  }
  - ],
  - "@babel/plugin-syntax-object-rest-spread"
  + ]
  ],
  "env": {
  "testing": {
  