Increase default memory for "active_config" shared dict.

This is related to 18F/api.data.gov#385

This should allow the default configuration to publish somewhere in the
neighborhood of 750-1500 API backends (up from 150-300) without running
into potential issues.

We still need to address the more underlying issue so the shard dict
operates in a safer manner (eg, by using set_safe as described in the
github issue).

config/default.yml

@@ -35,7 +35,7 @@ nginx:
     port: 14005
   shared_dicts:
     active_config:
-      size: 600k
+      size: 3m
     api_users:
       size: 3m
     dns_cache: