Drop support for ActiveModel 3.2

[tfausak]

Fixes #345.

[tfausak]

I would like to replace ActiveInteraction::Errors::Details with active_model-errors_details, but that gem doesn't work with Rails 5. I want to add something like this to our gem spec:

if spec.version_of 'active_model' < 5
  spec.add_dependency 'active_model-errors_details', '~> 1.3'
end

But I don't know how to do that. I don't even know if it's possible.

[AaronLasseigne]

I have no idea how to do that.

[tfausak]

If I had to guess, I'd say it's not possible. I poked around a little in Gem::Specification and Gem::Dependency and didn't find anything helpful.

I'm ok with leaving ActiveInteraction::Errors::Details in there. We can drop it when we drop support for Rails < 5 😉

[AaronLasseigne]

Good idea. No need to repeat the version numbers.

[AaronLasseigne]

[AaronLasseigne]

It looks like we can also drop the HashFilter code in the backport. It's Rails < 4.0.0.