Switch to codecov.io from coveralls

Coveralls doesn't care at all about branch coverage reporting, it just cares about lines (https://github.com/lemurheavy/coveralls-public/issues/31). [We are already choosing to generate branch coverage stats](https://github.com/pyca/cryptography/blob/master/.coveragerc#L2), but coveralls just ignores them. [codecov.io](http://codecov.io) incorporates branch data into their test coverage reporting. You can see an example [here using pyca/cryptography](https://codecov.io/github/frewsxcv/cryptography?ref=codecov) (keep in mind I disabled osx support when this ran).