Prepend "s." to stats JS link in $stats_footer

Stats JS link in $stats_footer needs "s." prepended to resolve.
https://github.com/Automattic/jetpack/blob/master/modules/stats.php#L155

Compare
http://s.stats.wordpress.com/e-201420.js
to
http://stats.wordpress.com/e-201420.js (current output)

From the looks of
http://en.wordpress.com/stats/
could also do http://s.stats.wordpress.com/w.js?21 format, but without knowing the inner workings or file naming conventions at work, included fix seems simplest option.

Cursory search through issues turned up https://github.com/Automattic/jetpack/issues/401 about Photon which seems unrelated, paging @skeltoac who seemed to know the most about wpcom and stats JS.