Remove rails-observers dependency

[domcleal]

The Audited::Sweeper class made limited use of the cache sweeping
functionality provided by rails-observer, mostly using it as a way to
implement Audit#before_create on the same class that was attached
around controller actions.

Since there is no released version of rails-observer that's compatible
with Rails 5.0 (and not in the foreseeable future without considerable
refactoring), the population of IPs, users and request UUIDs is replaced
by thread-local storage and the existing Audit#before_create methods.

This is a net code reduction and somewhat simplifies the "sweeper" by
removing the assumptions about how rails-observer works.

[thomasfedb]

Would be very keen to see this released.

[thomasfedb]

This as well as #324 have been merged into my fork (https://github.com/thomasfedb/audited). If there's enough interest I'm happy to cut a new minor release, although I'll need to change the gem name.

[domcleal]

Thanks @thomasfedb, I'd be interested in a new gem if audited itself or rails-observers aren't updated within a couple of weeks.

[thomasfedb]

@domcleal no worries. If you think of a decent gem name (I'm thinking trackback or metameta), or if there's anything that needs fixing then drop me a PR or create an Issue. I also see it as an opportunity to refactor and drop support for Ruby < 2.4 and Rails < 5.

[thomasfedb]

@domcleal also note that, from what I can see, rails-observers is fairly dead.