Visibility dirty tracking

[cjcolvar]

Prior to this PR, dirty tracking of visibility was implemented in a limited, custom way. #visibility_changed? with no arguments was the only method implemented. Switching to ActiveModel::Dirty makes dirty tracking support more standard and with more available methods (including #visibility_changed? with :from and :to keyword arguments).

Changes proposed in this pull request:

• Implement ActiveModel::Dirty for visibility
• Make child gems match parent hydra-head rails version dependencies
• Remove outdated handling of activemodel < 4.2 in unit specs

@samvera/hydra-head

[cjcolvar]

Testing in Hyrax showed that there is a small behavior change introduced by this change (at least in Hyrax). This change causes the access_control object of a work to be initialized instead of nil after initializing the work due to the after_initialize callback running. From what I can tell this does not lead to any appreciable change with nothing being persisted to Fedora.

[no-reply]

This change causes the access_control object of a work to be initialized instead of nil after initializing the work due to the after_initialize callback running. From what I can tell this does not lead to any appreciable change with nothing being persisted to Fedora.

it probably adds at least one HTTP HEAD request, since AF makes those calls when initializing objects. it's probably fair to consider that an AF issue, though.