Use protocol relative URL for OSM tiles

OpenStreetMap have enabled SSL on their services and website. Using protocol relative URLs for the tiles takes advantage of this work out of the box.

see: 
  - OSM blog: http://blog.openstreetmap.org/2014/02/11/osm-enhances-user-privacy/
  - specification: [IETF RFC 3986](http://www.ietf.org/rfc/rfc3986.txt) section 4.2