Update koa to version 2.2.0 🚀

[greenkeeperio-bot]

Hello lovely humans,

koa just published its new version 2.2.0.

State	Update 🚀
Dependency	koa
New version	2.2.0
Type	dependency

This version 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 koa.
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.

Do you have any ideas how I could improve these pull requests? Did I report anything you think isn’t right?
Are you unsure about how things are supposed to work?

There is a collection of frequently asked questions and while I’m just a bot, there is a group of people who are happy to teach me new things. Let them know.

Good luck with your project ✨

You rock!

🌴

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The new version differs by 221 commits .

• 9a9949f 2.2.0
• 55d1d9a travis: cache node_modules (#934)
• 4816cd7 :arrow-up: deps
• f841418 package: drop package.engines.node req to v6
• ebed04f 2.1.0
• e6539e1 Return middleware chain promise from callback() (#848)
• 65c130d readme docs++ (#932)
• e812339 docs: create v2 Migration document (#931)
• e9d7aba res: use http.ServerResponse._header when accessors exist (#930)
• 188c096 Add Troubleshooting documentation (#921)
• e452b68 feat: set err.headerSent before app error event emit (#919)
• 0c28d1f Fix async/await babel recommendation
• 909b829 Fix typo
• d740d9b Update the link to migration instructions (#916)
• 73e9e13 chore: unify badge's style (#914)

There are 221 commits in total. See the full diff.

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