Update prop-types to the latest version 🚀

[greenkeeper[bot]]

Version 15.6.0 of prop-types just got published.

Dependency	prop-types
Current Version	0.2.0
Type	dependency

The version 15.6.0 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 prop-types. 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:

[codecov[bot]]

Codecov Report

Merging #200 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #200   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files           9        9           
  Lines         241      241           
=======================================
  Hits          224      224           
  Misses         17       17

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Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8702799...666beef. Read the comment docs.

[coveralls]

Coverage remained the same at 91.62% when pulling 666beeffd5989eaae8d1e999f8b02f2b509c8b70 on greenkeeper/prop-types-15.6.0 into 8702799f9fc1ea30ab42ca0fdc8729b0d4e09011 on master.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.