Oh. You have a point there! I forgot this. So Should i continue by skipping this step like this?
cd /var/discourse
apt-get update
apt-get install libmysqlclient-dev mc -y
gem install mysql2 # mysql2-0.3.18 got installed
cd /var/www/discourse/script/import_scripts
bundle install
mcedit mybb.rb
RAILS_ENV=production ruby mybb.rb