mysql-only installation from source broken: Gem::LoadError: pg is not part of the bundle

Per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16471#note_55362312

i'm getting the following error after upgrading to current master branch (installation from sources, running MySQL)

rake aborted!
Gem::LoadError: pg is not part of the bundle. Add it to your Gemfile.
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:16:in `<top (required)>'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `require'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `block in require'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:240:in `load_dependency'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `require'
/home/git/gitlab/config/initializers/ar5_pg_10_support.rb:3:in `<top (required)>'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:268:in `load'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:268:in `block in load'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:240:in `load_dependency'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:268:in `load'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/engine.rb:652:in `block in load_config_initializer'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/notifications.rb:166:in `instrument'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/engine.rb:651:in `load_config_initializer'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/engine.rb:616:in `block (2 levels) in <class:Engine>'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/engine.rb:615:in `each'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/engine.rb:615:in `block in <class:Engine>'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:30:in `instance_exec'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:30:in `run'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:55:in `block in run_initializers'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:44:in `each'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:44:in `tsort_each_child'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/initializable.rb:54:in `run_initializers'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/application.rb:352:in `initialize!'
/home/git/gitlab/config/environment.rb:5:in `<top (required)>'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `require'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `block in require'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:240:in `load_dependency'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.10/lib/active_support/dependencies.rb:274:in `require'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/application.rb:328:in `require_environment!'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/railties-4.2.10/lib/rails/application.rb:457:in `block in run_tasks_blocks'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:62:in `block (2 levels) in define'
/home/git/gitlab/vendor/bundle/ruby/2.3.0/gems/rake-12.3.0/exe/rake:27:in `<top (required)>'

Removing config/initializers/ar5_pg_10_support.rb solved the issue

/cc @rtripault