use arel `master` and update activerecord to depend on arel `5.0.0`.

This is a follow-up to 3053fee954.

> The `master` branch should use the `master` branch.

Gemfile

@@ -12,7 +12,7 @@ gem 'bcrypt-ruby', '~> 3.1.2'
 gem 'jquery-rails', '~> 2.2.0'
 gem 'turbolinks'
 gem 'coffee-rails', '~> 4.0.0'
-gem 'arel', github: 'rails/arel', branch: '4-0-stable'
+gem 'arel', github: 'rails/arel'
 
 # This needs to be with require false to avoid
 # it being automatically loaded by sprockets

activerecord/activerecord.gemspec

@@ -24,5 +24,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'activesupport', version
   s.add_dependency 'activemodel',   version
 
-  s.add_dependency 'arel',                            '~> 4.0.0'
+  s.add_dependency 'arel',                            '~> 5.0.0'
 end