Use certifi provided certificates when downloading files

[mayeut]

As proposed by @jcrist, use certifi certificates when downloading files from host using the download function in cibuildwheel/util.py.
This behavior can be overridden using the SSL_CERT_FILE environment variable.

fix #452

[joerick]

Thanks for writing this, @mayeut. I'm +1 on this from a pragmatic standpoint. And considering requests does this, it doesn't seem like much of a risk either.

[mayeut]

Thanks for the review @joerick
Merging the approved PR then.