Fix OpenStreetMap URLs in examples

[rkaravia]

• Use https everywhere
• replace ${z}/${x}/${y} by {z}/{x}/{y}

In the OSM wiki,the tile server URLs are specified as:

https://a.tile.openstreetmap.org/${z}/${x}/${y}.png
https://b.tile.openstreetmap.org/${z}/${x}/${y}.png
https://c.tile.openstreetmap.org/${z}/${x}/${y}.png

See https://wiki.openstreetmap.org/wiki/Tile_servers

However, the ${x} notation was never part of the TileJSON specification which consistently uses {x}.

Fixes #18

[mapsam]

Thanks for the PR @rzoller! This looks great. The usage of ${z} must have been taken directly from the OSM wiki, as you point out. Since the tilejson spec does not use this notation, removing clears it up. Fixing all previous versions seems like a safe choice as well.

Let’s disregard the Travis failure, which was enabled without an actual .travis.yml file - I’ve disabled this since we don’t have any tests.