Update rollup to the latest version 🚀 (#58)

## The devDependency [rollup](https://github.com/rollup/rollup) was updated from `0.66.6` to `0.67.0`.
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.

---

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

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package.json

@@ -57,7 +57,7 @@
     "react-docgen": "^3.0.0-beta12",
     "react-dom": "^16.5.0",
     "react-test-renderer": "^16.5.0",
-    "rollup": "^0.66.0",
+    "rollup": "^0.67.0",
     "rollup-plugin-babel": "^4.0.2"
   },
   "greenkeeper": {