Make railties an optional dependency

[ggpasqualino]

Purpose

Make railties an optional dependency in order to use active_model_serializers in non rails projects again.

Related GitHub issues

#1921

[mention-bot]

@ggpasqualino, thanks for your PR! By analyzing the annotation information on this pull request, we identified @bf4, @dubadub and @steveklabnik to be potential reviewers

[richmolj]

I'm in favor of this change. It looks like this PR by @bf4 moved us from rails as a development dependency to a runtime dependency, but reading the commit/PR this seems like it might have been unintended.

@bf4 is that correct?

(side note: if we go in the direction of deprecating ActiveModelSerializers::Model activemodel could move to a development dependency as well, I think)

[richmolj]

Also, just a heads up we are already only requiring the railtie if Rails is defined so no worries there.

@ggpasqualino even though this is a one-line change, I could see an entry in the changelog being helpful.

[ggpasqualino]

@richmolj thank you. I will check the changelog and update the PR.

Sorry about this mess. I didn't realize that when I created #1921 because I was looking at the wrong version of the file, then I realised that in master you are already only requiring the railtie if Rails is defined and created this Pull Request.

[bf4]

👍