Change cache_buster url argument

Presumably fixes https://github.com/azavea/OTM2/issues/659 but I could
not repoduce the issue, which was only shown in production with load
balanced tile servers.

Adam was able to make this bug disappear by adding a render_cache.clear
to every tile request. Searching the Windshaft code, I found the tile
renderer caching respects a query string argument named cache_buster

https://github.com/azavea/Windshaft/blob/master/lib/windshaft/render_cache.js#L236
Merge pull request #26 from jwalgran/fix/cache-buster

Change cache_buster url argument