Update rollup to the latest version 🚀 (#43)

## The devDependency [rollup](https://github.com/rollup/rollup) was updated from `0.67.4` to `0.68.0`.
This version is **not covered** by your **current version range**.

If you don’t accept this pull request, your project will work just like it did before. However, you might be missing out on a bunch of new features, fixes and/or performance improvements from the dependency update.

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[Find out more about this release](https://github.com/rollup/rollup).

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package.json

@@ -68,7 +68,7 @@
     "react": "^16.5.2",
     "react-dom": "^16.5.2",
     "react-test-renderer": "^16.5.2",
-    "rollup": "^0.67.0",
+    "rollup": "^0.68.0",
     "rollup-plugin-babel": "^4.0.3",
     "rollup-plugin-node-resolve": "^4.0.0"
   },