message: move koa from dependencies to devDependencies
  also be less strict on the peer dependency version and accept any koa version 2
diff: "koa"
  ],
  "peerDependencies": {
  - "koa": "^2.3.0"
  + "koa": "^2"
  },
  "devDependencies": {
  "@types/node": "9.4.0",
  "rimraf": "2.6.2",
  - "typescript": "2.6.2"
  + "typescript": "2.6.2",
  + "koa": "^2.5.1"
  },
  "typings": "dist/index.d.ts",
  "typescript": {
  "definition": "dist/index.d.ts"
  },
  "dependencies": {
  - "graphql-playground-html": "1.5.4",
  - "koa": "^2.4.1"
  + "graphql-playground-html": "1.5.4"
  }
  }
  