Ensure FreeBSD Ruby install points at valid root CA cert

On FreeBSD the Ruby install's SSL points at `/etc/ssl/cert.pem` as the default root CA cert. This is actually the location of an optional symlink that the `ca_root_nss` port creates. We can't guarantee this optional symlink will exist so we'll just point `SSL_CERT_FILE` at the actual location of the root CA cert.

/cc @opscode-cookbooks/release-engineers @sersut