Update rollup to the latest version 🚀 (#2404)

* chore(package): update rollup to version 1.9.1

* chore(package): update lockfile package-lock.json

Author: greenkeeper[bot]

package-lock.json

@@ -64,7 +64,7 @@
     "local-web-server": "2.6.1",
     "markdown": "0.5.0",
     "promise-polyfill": "8.1.0",
-    "rollup": "1.9.0",
+    "rollup": "1.9.1",
     "rollup-plugin-babel": "4.3.1",
     "rollup-plugin-buble": "^0.19.6",
     "rollup-plugin-node-resolve": "4.2.1",