Fix Codecov integration

Follow on from https://github.com/python-pillow/Pillow/pull/3913#issuecomment-504881781.

Again the Codecov integration isn't reporting on PRs:

![image](https://user-images.githubusercontent.com/1324225/60488622-165c5000-9cab-11e9-94a3-29c785e066a8.png)

https://github.com/python-pillow/Pillow/pull/3933

Ping @aclark4life. Perhaps removing and re-adding it may help?