Redmine on Ubuntu 13.10 with Apache2 and Passenger

After a fresh Ubuntu 13.10 install, I encountered the following error [Fri Nov 01 20:06:50.239664 2013] [core:notice] [pid 59440] AH00094: Command line: ‘/usr/sbin/apache2’ [ pid=59448 thr=19153000 file=utils.rb:176 time=2013-11-01 20:06:51.996 ]: *** Exception LoadError in PhusionPassenger::ClassicRails::ApplicationSpawner (Please install the mysql adapter: `gem install activerecord-mysql-adapter` (cannot load such file — mysql)) (process 59448, thread #): I tried …