Tests: Explicitly specify rails as a dev. dependency

spec/dummy_app needs rails. Technically, it only needs actionpack,
but in case that changes in the future, I think we should just
have rails as an explicit dependency.

You may ask, if dummy_app has always needed actionpack, and our
gemspec didn't explicitly specify the later, how did it ever work?
It looks like we were getting actionpack as a transitive dependency.

Appraisals

@@ -8,13 +8,14 @@
 # > appraisal. If something is specified in both the Gemfile and an appraisal,
 # > the version from the appraisal takes precedence.
 # > https://github.com/thoughtbot/appraisal
-
+#
+#
 appraise "ar-5.2" do
-  gem "activerecord", "~> 5.2.4"
+  gem "rails", "~> 5.2.4"
   gem "rails-controller-testing", "~> 1.0.2"
 end
 
 appraise "ar-6.0" do
-  gem "activerecord", "~> 6.0.3"
+  gem "rails", "~> 6.0.3"
   gem "rails-controller-testing", "~> 1.0.3"
 end

gemfiles/ar_5.2.gemfile

@@ -2,7 +2,7 @@
 
 source "https://rubygems.org"
 
-gem "activerecord", "~> 5.2.4"
+gem "rails", "~> 5.2.4"
 gem "rails-controller-testing", "~> 1.0.2"
 
 gemspec path: "../"

gemfiles/ar_6.0.gemfile

@@ -2,7 +2,7 @@
 
 source "https://rubygems.org"
 
-gem "activerecord", "~> 6.0.3"
+gem "rails", "~> 6.0.3"
 gem "rails-controller-testing", "~> 1.0.3"
 
 gemspec path: "../"

paper_trail.gemspec

@@ -31,7 +31,7 @@ has been destroyed.
   # https://www.ruby-lang.org/en/news/2019/10/02/ruby-2-4-9-released/
   s.required_ruby_version = ">= 2.4.0"
 
-  # We no longer specify a maximum rails version.
+  # We no longer specify a maximum activerecord version.
   # See discussion in paper_trail/compatibility.rb
   s.add_dependency "activerecord", ::PaperTrail::Compatibility::ACTIVERECORD_GTE
   s.add_dependency "request_store", "~> 1.1"
@@ -41,6 +41,12 @@ has been destroyed.
   s.add_development_dependency "ffaker", "~> 2.11"
   s.add_development_dependency "generator_spec", "~> 0.9.4"
   s.add_development_dependency "memory_profiler", "~> 0.9.14"
+
+  # For `spec/dummy_app`. Technically, we only need `actionpack` (as of 2020).
+  # However, that might change in the future, and the advantages of specifying a
+  # subset (e.g. actionpack only) are unclear.
+  s.add_development_dependency "rails", ::PaperTrail::Compatibility::ACTIVERECORD_GTE
+
   s.add_development_dependency "rake", "~> 13.0"
   s.add_development_dependency "rspec-rails", "~> 4.0"
   s.add_development_dependency "rubocop", "~> 0.89.1"