fix(deps): update dependency koa-convert to v2

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
koa-convert	^1.2.0 -> ^2.0.0

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Release Notes

gyson/koa-convert (koa-convert)

v2.0.0

Compare Source

• update: ready to next release (#​13) 290ca57
• update build status link 481010e
• add koa-router and koa-66 in README a2227eb
• update koa-route link 18f8465
• update Note in README.md d18b0c5
• 1.2.0 da39a52
• inherit original name for convert.back fe140c0
• Merge pull request #​1 from 75lb/master d0d6649
• inherit the original middleware's name 2339659
• 1.1.0 47548f5
• update README.md ce4158a
• add convert.back() 1d58b7c
• add node v5 in .travis.yml e18b275
• 1.0.0 8eca268
• add npm version e61930a
• add test for migration snippet 87fc958
• update README 1ee4700
• fix typo c30e79d
• add convert.compose() a32f440
• change error message 27e50c7
• more test fe32dd4
• add build status 26a740e
• add travis 49bbbc6
• use standard e6628be
• add url link d191520
• update README eda2ba9
• fix typo 3e69dd9
• add keywords aaa30a5
• update Usage in README 6cb421c
• add implementation, readme and test c1bdd75

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