Use Arel master when using ActiveRecord master

(Transplant from with_model/with_model@cab41ed225a5d1f1fb65e51edc5673f2583285bb)

Gemfile

@@ -5,8 +5,14 @@ gemspec
 gem 'pg', :platform => :ruby
 gem "activerecord-jdbcpostgresql-adapter", ">= 1.3.1", :platform => :jruby
 
-gem "activerecord", ENV["ACTIVE_RECORD_VERSION"] if ENV["ACTIVE_RECORD_VERSION"]
-gem "activerecord", :github => "rails", :branch => ENV["ACTIVE_RECORD_BRANCH"] if ENV["ACTIVE_RECORD_BRANCH"]
+if ENV['ACTIVE_RECORD_BRANCH']
+  gem 'activerecord', :git => 'https://github.com/rails/rails.git', :branch => ENV['ACTIVE_RECORD_BRANCH']
+  gem 'arel', :git => 'https://github.com/rails/arel.git' if ENV['ACTIVE_RECORD_BRANCH'] == 'master'
+end
+
+if ENV['ACTIVE_RECORD_VERSION']
+  gem 'activerecord', ENV['ACTIVE_RECORD_VERSION']
+end
 
 group :development do
   gem 'guard-rspec', :require => false