Unify shields type

[Kubuxu]

Each service provider uses a bit different shield type, we can unify them to be all from shields.io and to look the same, this might be more of an issues to standard readme, but I am not sure.

https://img.shields.io/codecov/c/github/ipfs/go-ipfs.svg?style=flat-square

[RichardLitt]

This issue belongs here.

I agree about unifying shield type. Let's make them all flat-square, makes it easier, and use shields.io for all of them.