Update koa-static-cache to version 3.1.3 🚀

[greenkeeperio-bot]

Hello :wave:

:rocket::rocket::rocket:

koa-static-cache just published its new version 3.1.3, 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-cache – 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 5 commits .

• 94e5938 Release 3.1.3
• afc218f Merge pull request #43 from teohhanhui/fix-mtime
• 7aefae3 Merge pull request #44 from koajs/node4_iojs
• 6885e08 run tests on node 4.x.x instead of iojs
• 7b12f6f Fix broken mtime comparison

See the full diff.

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