Allow incompatible versions of ActiveRecord

[jaredbeck]

Re: #1213

For advanced users only. A warning is produced. See discussion
in paper_trail/compatibility.rb

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[jaredbeck]

@rafaelfranca does this work for you?

[Edouard-chin]

Thanks for looking into this @jaredbeck , much appreciated 🙇 .

I think outputting a warning each time the gem is loaded is not very user friendly (even if we can disable it with a special env). It would be better if the warning was output if papertrail get installed for the first time or whenever activerecord gets updated through a bundler update.

We can achieve that by using a small rubygem plugin and the Gem.post_install hook https://github.com/Edouard-chin/paper_trail/pull/new/ec-allow-incompatible-ar

[rafaelfranca]

I'm very happy with this change.

[jaredbeck]

Thanks for the reviews.

It would be better if the warning was output if papertrail get installed for the first time or whenever activerecord gets updated through a bundler update.

Interesting idea, Edouard, but I'm not comfortable with it, sorry. Once we give up the safety net of an upper-bound, normal users will accidentally install incompatible versions. It will happen frequently. I want to be certain that the warning is seen. We have to take care of beginners and normal users.