You always want it to be // rather than http://

The reason is that this allows the browser to adapt to the protocol that the site is being serve from (protocol meaning http or https).

If your site is https (secure protocol) then having this link would trigger errors to the user, warnings that not all content is secure.

using // allows the browser to "fill in" http or https based on whether or not YOUR page is being served as http or https.

In our case, we aren't using https so it won't matter, but in an ideal world we would be able to change to https without changing any code.