Update Appraisals file to fetch latest versions

Bundler seemed to be having a hard time resolving to a later version
of the Rails 5.0.x series so simplify the gemfiles to give bundler
more freedom to resolve things.

Appraisals

@@ -1,19 +1,13 @@
 appraise "rails-4-2" do
-  gem "railties",    "~> 4.2.0"
-  gem "actionpack",  "~> 4.2.0"
-  gem "activemodel", "~> 4.2.0"
+  gem "rails",    "~> 4.2.0"
 end
 
 if RUBY_VERSION >= "2.2.2"
   appraise "rails-5-0" do
-    gem "railties",    "~> 5.0.0"
-    gem "actionpack",  "~> 5.0.0"
-    gem "activemodel", "~> 5.0.0"
+    gem "rails",    ">= 5.0.0", "< 5.1"
   end
 
   appraise "rails-5-1" do
-    gem "railties",    ">= 5.1.0", "< 5.2"
-    gem "actionpack",  ">= 5.1.0", "< 5.2"
-    gem "activemodel", ">= 5.1.0", "< 5.2"
+    gem "rails",    ">= 5.1.0", "< 5.2"
   end
 end

gemfiles/rails_4_2.gemfile

@@ -6,8 +6,6 @@ gem "rake"
 gem "mocha", require: false
 gem "appraisal"
 gem "pry"
-gem "railties", "~> 4.2.0"
-gem "actionpack", "~> 4.2.0"
-gem "activemodel", "~> 4.2.0"
+gem "rails", "~> 4.2.0"
 
 gemspec path: "../"

gemfiles/rails_5_0.gemfile

@@ -6,8 +6,6 @@ gem "rake"
 gem "mocha", require: false
 gem "appraisal"
 gem "pry"
-gem "railties", "~> 5.0.0"
-gem "actionpack", "~> 5.0.0"
-gem "activemodel", "~> 5.0.0"
+gem "rails", ">= 5.0.0", "< 5.1"
 
 gemspec path: "../"

gemfiles/rails_5_1.gemfile

@@ -6,8 +6,6 @@ gem "rake"
 gem "mocha", require: false
 gem "appraisal"
 gem "pry"
-gem "railties", ">= 5.1.0", "< 5.2"
-gem "actionpack", ">= 5.1.0", "< 5.2"
-gem "activemodel", ">= 5.1.0", "< 5.2"
+gem "rails", ">= 5.1.0", "< 5.2"
 
 gemspec path: "../"