Reduce gem dependencies to railties

Currently the gem depends on the entirety of `rails`, which adds some
"dependency weight" to projects that only need a subset of Rails
functionality. This changes the gem to depend on `railties` instead,
which covers all of the core Rails functionality Loaf needs to function.

Appraisals

@@ -1,4 +1,4 @@
-rails_versions = [
+railties_versions = [
   %w[3.2 3.2.22.5],
   %w[4.0 4.0.13],
   %w[4.1 4.1.16],
@@ -9,11 +9,11 @@ rails_versions = [
   %w[6.0 6.0.2],
 ]
 
-rails_versions.each do |(version, rails)|
+railties_versions.each do |(version, railties)|
   gem_version = Gem::Version.new(version)
 
-  appraise "rails#{version}" do
-    gem "rails", "~> #{rails}"
+  appraise "railties#{version}" do
+    gem "railties", "~> #{railties}"
     gem "capybara", "~> 2.18.0"
 
     if gem_version <= Gem::Version.new("4.0")

gemfiles/rails3.2.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 3.2.22.5"
+gem "railties", "~> 3.2.22.5"
 gem "test-unit", "~> 3.0"
 
 group :metrics do

gemfiles/rails4.0.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 4.0.13"
+gem "railties", "~> 4.0.13"
 gem "test-unit", "~> 3.0"
 
 group :metrics do

gemfiles/rails4.1.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 4.1.16"
+gem "railties", "~> 4.1.16"
 
 group :metrics do
   gem "coveralls", "0.8.23"

gemfiles/rails4.2.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 4.2.10"
+gem "railties", "~> 4.2.10"
 
 group :metrics do
   gem "coveralls", "0.8.23"

gemfiles/rails5.0.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 5.0.7"
+gem "railties", "~> 5.0.7"
 
 group :metrics do
   gem "coveralls", "0.8.23"

gemfiles/rails5.1.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 5.1.6"
+gem "railties", "~> 5.1.6"
 
 group :metrics do
   gem "coveralls", "0.8.23"

gemfiles/rails5.2.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.3.13", platforms: :ruby
-gem "rails", "~> 5.2.0"
+gem "railties", "~> 5.2.0"
 
 group :metrics do
   gem "coveralls", "0.8.23"

gemfiles/rails6.0.gemfile

@@ -9,7 +9,7 @@ gem "capybara", "~> 2.18.0"
 gem "rspec-rails", "~> 3.9.0"
 gem "public_suffix", "~> 2.0.5"
 gem "sqlite3", "~> 1.4.2", platforms: :ruby
-gem "rails", "~> 6.0.2"
+gem "railties", "~> 6.0.2"
 
 group :metrics do
   gem "coveralls", "0.8.23"

loaf.gemspec

@@ -25,8 +25,9 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.required_ruby_version = ">= 1.9.3"
 
-  spec.add_dependency "rails", ">= 3.2"
+  spec.add_dependency "railties", ">= 3.2"
 
   spec.add_development_dependency "bundler", ">= 1.5"
+  spec.add_development_dependency "rails", ">= 3.2"
   spec.add_development_dependency "rake"
 end