Fix up flake8 H302

CI fails in PR #555 which is irrelevant to it. So I send another PR to fix.
It's strange that it didn't fail in master's CI.

https://travis-ci.org/docker/docker-registry/jobs/34466985#L785
```
./docker_registry/app.py:19:1: H302  import only modules.'from flask.ext.cors import CORS' does not import a module
```