This should speed it up a bit, as brew will download a pre-compiled "bottle", whereas pyenv compiles python from source.
The current 'python3' formula on homebrew is 3.6.0, so let's use that.
Previously we were using a separate Travis build that only run coverage with a single Python version (3.5).
Now, we run coverage with all python versions, and upload a combined coverage data to Codecov.io.