Wikia.InstantGlobals doesn't supply the proper Last-Modified header

https://wikia-inc.atlassian.net/browse/PLATFORM-298

Emit ``Last-Modified`` response header with the current timestamp. It will cause CDN to update the cache entry on every miss.

@gabrys / @Wikia/platform-team 
``ResourceLoaderHooks::normalizeTimestamp`` method normalizes timestamp returned by startup module to 15 minute buckets.