Upgrade gatsby-plugin-offline in tests

Author: sidharthachatterjee

e2e-tests/production-runtime/package.json

@@ -7,7 +7,7 @@
     "cypress": "^3.1.0",
     "gatsby": "^2.0.118",
     "gatsby-plugin-manifest": "^2.0.17",
-    "gatsby-plugin-offline": "^2.0.23",
+    "gatsby-plugin-offline": "^3.0.0",
     "gatsby-plugin-react-helmet": "^3.0.6",
     "gatsby-seo": "^0.1.0",
     "glob": "^7.1.3",