Gemfile

database = begin
  File.read(File.dirname(__FILE__) + '/config/database.yml').scan(/production.+?adapter\W+(\w+)/im).join
rescue
  nil
end

case database
when /postgres/, /pg/
  gem 'pg', :require => false
when /sqlite/
  gem 'sqlite3-ruby', :require => false
else
  gem 'mysql', :require => false
end

All the credit for this goes to Dimitrij Denissenko. The original code is here.