As of December 2014, Paul Irish’s blog on protocol-relative URLs says:
2014.12.17: Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique is now an anti-pattern. If the asset you need is available on SSL, then always use the
https://asset.
Unless you have specific performance concerns (such as the slow mobile network mentioned in Zakjan’s answer) you should use https:// to protect your users.