wrong graphql-core version installed

Hey,

I'm testing sonic-graphql and I've noticed that when I install it, it tries to install `graphql-core>=2.1rc1` but installs `graphql-core 2.0` instead https://pastebin.com/raw/BhAyEWCd

which then makes sanic-graphql fail on :
```
from graphql import get_default_backend
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
in ()
----> 1 from graphql import get_default_backend

ImportError: cannot import name 'get_default_backend'
```

If instead I first install `graphql-core==2.1rc1` I don't have any issue