Upgrade acts-as-taggable-on from 3.5.0 to 4.0.0.
What does this MR do?
Upgrades the acts-as-taggable-on gem from 3.5.0 to 4.0.0.
Changelog: https://github.com/mbleigh/acts-as-taggable-on/blob/master/CHANGELOG.md#400--2016-08-08
Are there points in the code the reviewer needs to double check?
That this works without the migrations, I don't believe the upgrade from 3.5.0 to 4.0.0 requires any migrations since UPGRADING.md hasn't been updated since 3.5.0 was released. It's kind of unclear, unfortunately.
What are the relevant issue numbers?
Fixes #22082 (closed). Working toward Rails 5: #14286 (moved).
cc: @dzaporozhets