Prepare for Rails 5, don't simply include Rails be explicit

Gemfile

@@ -12,9 +12,18 @@ if rails_master?
   gem 'rails-observers', git: 'https://github.com/rails/rails-observers.git'
   gem 'seed-fu', git: 'https://github.com/SamSaffron/seed-fu.git', branch: 'discourse'
 else
-  gem 'rails', '~> 4.2'
+  # Rails 5 is going to ship with Action Cable, we have no use for it as
+  # we already ship MessageBus, AC introduces dependencies on Event Machine,
+  # Celluloid and Faye Web Sockets.
+  gem 'activesupport', '~> 4.2'
+  gem 'actionpack'
+  gem 'activerecord'
+  gem 'actionmailer'
+  gem 'activejob'
+  gem 'railties'
   gem 'rails-observers'
   gem 'seed-fu', '~> 2.3.5'
+  gem 'sprockets-rails'
 end
 
 gem 'mail'

Gemfile.lock

@@ -199,8 +199,8 @@ GEM
     metaclass (0.0.4)
     method_source (0.8.2)
     mime-types (2.6.2)
-    mini_portile (0.6.2)
-    minitest (5.8.0)
+    mini_portile2 (2.0.0)
+    minitest (5.8.3)
     mocha (1.1.0)
       metaclass (~> 0.0.1)
     mock_redis (0.15.2)
@@ -212,8 +212,8 @@ GEM
     mustache (1.0.2)
     nenv (0.2.0)
     netrc (0.10.3)
-    nokogiri (1.6.6.4)
-      mini_portile (~> 0.6.0)
+    nokogiri (1.6.7.1)
+      mini_portile2 (~> 2.0.0.rc2)
     nokogumbo (1.4.1)
       nokogiri
     oauth (0.4.7)
@@ -441,7 +441,12 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  actionmailer
+  actionpack
   active_model_serializers (~> 0.8.3)
+  activejob
+  activerecord
+  activesupport (~> 4.2)
   annotate
   aws-sdk
   babel-transpiler
@@ -499,9 +504,9 @@ DEPENDENCIES
   r2 (~> 0.2.5)
   rack-mini-profiler
   rack-protection
-  rails (~> 4.2)
   rails-observers
   rails_multisite
+  railties
   rake
   rb-fsevent
   rb-inotify (~> 0.9)
@@ -527,6 +532,7 @@ DEPENDENCIES
   simplecov
   sinatra
   spork-rails
+  sprockets-rails
   stackprof
   therubyracer
   thin