Make sure cache key is always the same

This could cause corrupt cache data because depending on the
array elements' order, stuff is written to/read from different
cache keys.

I discovered this by running FlowPopulateLinksTables.php, which
ended up reading from one key and writing to another, and
inconsistently writing to different keys.

enwiki:flow_ref:wiki:by-revision:v2:rjqy0zy9gglw2ltm:post:4.4
enwiki:flow_ref:wiki:by-revision:v2:post:rjqy0zy9gglw2ltm:4.4

Change-Id: Idf1b798cd79c9bafb908d3426542e71ec244745a