remove unused dependency

Author: bertho-zero

package.json

@@ -197,7 +197,6 @@
     "lighthouse": "^1.5.1",
     "mocha": "^3.2.0",
     "node-sass": "^4.5.0",
-    "offline-plugin": "^4.8.0",
     "phantomjs-polyfill": "^0.0.2",
     "phantomjs-prebuilt": "^2.1.14",
     "piping": "^1.0.0-rc.4",