Update jest to the latest version 🚀

[greenkeeper[bot]]

Version 21.0.0 of jest just got published.

Dependency	jest
Current Version	20.0.4
Type	devDependency

The version 21.0.0 is not covered by your current version range.

Without accepting this pull request your project will work just like it did before. There might be a bunch of new features, fixes and perf improvements that the maintainers worked on for you though.

I recommend you look into these changes and try to get onto the latest version of jest. Given that you have a decent test suite, a passing build is a strong indicator that you can take advantage of these changes by merging the proposed change into your project. Otherwise this branch is a great starting point for you to work on the update.

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Not sure how things should work exactly?

There is a collection of [frequently asked questions](https://greenkeeper.io/faq.html) and of course you may always [ask my humans](https://github.com/greenkeeperio/greenkeeper/issues/new).

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Your Greenkeeper Bot :palm_tree:

[greenkeeper[bot]]

Version 21.0.1 just got published.

Update to this version instead 🚀

[codecov[bot]]

Codecov Report

Merging #39 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #39   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          20     20           
  Lines         172    172           
  Branches       41     41           
=====================================
  Hits          172    172

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