Fix for SSL_CERT_FILE in cygwin

The vagrant aws plugin fails in cygwin because the SSL_CERT_FILE path is a cygwin path. https://github.com/mitchellh/vagrant-aws/issues/184

I assume like other paths for ruby we'd want this to be changed back to a windows path.

I'd be happy to add some tests for this but I'm not sure what is expected for this project.