Rails 5

[toastercup]

This issue exists to track Rails 5 upgrade feasability. Rails 5 is now stable, so we should start evaluating our dependent libraries over the next couple months:

• acts-as-taggable-on: master branch contains stable 5.x support, release imminent
• sprockets: Does not yet support Rails 5
• sidekiq + sinatra: Sidekiq requires Sinatra for its management interface, which relies on an old version of Rack that is not supported by Rails 5.
• redis-rails: 5.x support present in 5.0.0.pre version of this gem
• paranoia: Very alpha-looking 5.x support present in master branch
• rails-observers: 5.x support present in master branch

Once we can tick off all these libraries as stable/working, we can upgrade! During upgrading, we must follow this guide, as a decent bit has changed. We should also step through the release notes as a team.

[toastercup]

Closed by #382, #384 and #388