Update acts-as-taggable-on from 3.0.1 to 3.1.1

* This fixes the issue where the Mapping.with_traffic_summary scope
  would not chain with Mapping.tagged_with, generating invalid SQL when
  a `count` was required (falling over on `any?` and the like)
* While we initially patched AATO ourselves, it turns out a fix was
  already merged for AATO 3.1.0
  (https://github.com/mbleigh/acts-as-taggable-on/pull/417)
* 3.1.1 also had a tag counting performance improvement, which in
  truth doesn't do much for us but requires a migration