Windows: make ruby aware of our CA bundle

On Windows, the *net/http* library of the Vagrant's embedded ruby does **not** check the validity of an SSL certificate during a TLS handshake. This will cause issues to providers that use https and SSL certificates for their API calls.

For fixing this issue, we can make ruby aware of our certificate authority bundle by setting `SSL_CERT_FILE`.

Here is a more detailed explanation of the problem:
https://gist.github.com/fnichol/867550