Use relative URL when loading OSM tiles

This avoids mixed content warnings and gives us our
padlock back on Chromium and Firefox.

This also switches the domain for the tile servers from osm.org to
openstreetmap.org because the certificates do not mention osm.org
and hence using this domain the tiles fail to load.

app/views/conference/_venue_map.html.haml

@@ -5,7 +5,7 @@
     // create a map in the "map" div, set the view to a given place and zoom
     var map = L.map('map', { scrollWheelZoom: false }).setView([#{@conference.venue.latitude}, #{@conference.venue.longitude}], 11);
     // add an OpenStreetMap tile layer
-    L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
+    L.tileLayer('//{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
       attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
       maxZoom: 18
     }).addTo(map);