upgrade all the things (upgrade to Rails 5)

[jipiboily]

We need to upgrade Monologue properly.

• It needs to run at least on Rails 5, ideally with Rails 5.1 too.
• It needs to have a working test suite with about the same coverage (not in % but in terms of features covered)
• it needs to be possible to upgrade from an older version to the latest one without anything else than a bundle update monologue and running migrations.

I can offer $100 for anyone completing this with high quality work while having in mind it's open source and our current users needs to be able to upgrade smoothly.