Remove :idc_cached_nil

[fbogsany]

@dylanahsmith : "I think the only thing that prevented us from storing nil was that cache.read would return nil for a cache miss as well as if the value cached was nil. I think we may even be able to get rid of map_cached_nil_for now that we don't use cache.read."

Follow-up from #154

Note that support for cache.read was re-added in #161

[dylanahsmith]

If you still want to remove :idc_cached_nil, then you could replace cache.read with cache.read_multi for a single key. Otherwise, we could just close this issue now that we are supporting ActiveSupport::Cache::MemoryStore.