feat: make `deep_merge` a soft-requirement

[zzak]

The goal here is to remove deep_merge and rely entirely on ActiveSupport's Hash#deep_merge implementation.

This does mean that if you want to keep the existing behavior, or rely on DeepMerge's specific options, you need to add the following to your Gemfile:

gem 'deep_merge', '~> 1.2', '>= 1.2.1'

---

@pkuczynski Opening this as a proof-of-concept, would you be willing to take on this type of change?

I decided not to touch the config methods, even though they are no-op when deep_merge gem isn't included. Wanted to get feedback first, and I figure the warning is enough.