Change explicitly `http://` urls to `//`

This allows Cesium and examples to run properly on https connecitons.  This is needed for us to allow for https connections to cesiumjs.org.
Merge remote-tracking branch 'origin/master' into https
Changes after review.
Merge remote-tracking branch 'origin/master' into https
More changes after review.
Fix Sandcastle examples missed with last submit.
Merge pull request #1608 from AnalyticalGraphicsInc/https

Change explicitly `http://` urls to `//`