Update jest2 to the latest version 🚀 (#28)

## There have been updates to the *jest2* monorepo: 

- The `devDependency` [expect](https://github.com/facebook/jest) was updated from `23.6.0` to `24.0.0`.

These versions are **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.

This monorepo update includes releases of one or more dependencies which all belong to the [jest2 group definition](https://github.com/greenkeeperio/monorepo-definitions).

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

<details>
  <summary>FAQ and help</summary>

  There is a collection of [frequently asked questions](https://greenkeeper.io/faq.html). If those don’t help, you can always [ask the humans behind Greenkeeper](https://github.com/greenkeeperio/greenkeeper/issues/new).
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Your [Greenkeeper](https://greenkeeper.io) bot 🌴

package.json

@@ -43,7 +43,7 @@
     "eslint-plugin-import": "^2.14.0",
     "eslint-plugin-jsx-a11y": "^6.1.1",
     "eslint-plugin-react": "^7.11.0",
-    "expect": "^23.6.0",
+    "expect": "^24.0.0",
     "mocha": "^5.1.1",
     "react": "^16.3.2",
     "react-dom": "^16.3.2",