Merge branch 'blackst0ne-rails5-update-ar5-pg-10-support-initializer' into 'master'

[Rails5] Update ar5_pg_10_support initializer

See merge request gitlab-org/gitlab-ce!17826

config/initializers/ar5_pg_10_support.rb

-raise "Vendored ActiveRecord 5 code! Delete #{__FILE__}!" if ActiveRecord::VERSION::MAJOR >= 5
-
-if Gitlab::Database.postgresql?
+# Remove this file when upgraded to rails 5.0.
+if !Gitlab.rails5? && Gitlab::Database.postgresql?
   require 'active_record/connection_adapters/postgresql_adapter'
   require 'active_record/connection_adapters/postgresql/schema_statements'