[Rails5] Add FALSE_VALUES constant to Service#boolean_accessor

In Rails 5.0 the `ActiveRecord::ConnectionAdapters::Column::TRUE_VALUES`
constant has been removed [1] and the remaining `FALSE_VALUES` constant
has been moved to `ActiveModel::Type::Boolean` [2]

[1]: https://github.com/rails/rails/commit/a502703c3d2151d4d3b421b29fefdac5ad05df61
[2]: https://github.com/rails/rails/commit/9cc8c6f3730df3d94c81a55be9ee1b7b4ffd29f6

app/models/service.rb

@@ -206,7 +206,11 @@ class Service < ActiveRecord::Base
     args.each do |arg|
       class_eval %{
         def #{arg}?
-          ActiveRecord::ConnectionAdapters::Column::TRUE_VALUES.include?(#{arg})
+          if Gitlab.rails5?
+            !ActiveModel::Type::Boolean::FALSE_VALUES.include?(#{arg})
+          else
+            ActiveRecord::ConnectionAdapters::Column::TRUE_VALUES.include?(#{arg})
+          end
         end
       }
     end