Add graphql@15 as a valid peer dep
```
npm WARN koa-graphql@0.8.0 requires a peer of graphql@^0.12.0 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies 
yourself.
npm WARN express-graphql@0.7.1 requires a peer of graphql@^0.12.0 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.
```