mysql driver is no longer bundled w/ rails. you'll need to install it yourself w/

sudo gem install mysql

however, on ubuntu (heron) this won't work. issue these commands first

sudo apt-get install libmysql-ruby libmysqlclient-dev

if libmysqlclient-dev fails... try libmysqlclient15-dev

then run

sudo gem install mysql

Just finishing up brewing up some fresh ground comments...