Update koa-static to version 1.5.1 🚀

[greenkeeperio-bot]

Hello :wave:

:rocket::rocket::rocket:

koa-static just published its new version 1.5.1, which is not covered by your current version range.

If this pull request passes your tests you can publish your software with the latest version of koa-static – otherwise use this branch to work on adaptions and fixes.

Happy fixing and merging :palm_tree:

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The new version differs by 7 commits .

• aa212b9 1.5.1
• 4f76c53 fix v1.4.x → 1.5.0 broken
• 6614586 1.5.0
• acbb784 :arrow_up: bump devDeps
• 64c4d58 add tests for option - format
• 0e70310 :arrow_up: koa-send@2
• 2f394a5 travis: test node v1~v4

See the full diff.

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