Define active_support dependencies in production.

[tigris]

When using the datadog integration, it seems active_support is required. This can be seen here.

I see there is a helpful error message above if you don't include dogstatsd-ruby, so perhaps the fix here is to simply add the same check for active_support/subscriber? I only mention this becaus I had some trouble just now tracking down what it was in my chain that needed active support.

Lastly, I can see that a dev dependency for this is set to < 6.1. Would this be the same in prod for the above-mentioned code? As I'm not a consumer of active_support personally I can't easily tell. I do however note that another gem (database_cleaner to be exact) wants to make use of activesupport >= 7, so these requirements might get out of hand if that's true?