Ruby MySQL ActiveRecord::ConnectionNotEstablished

Trying my first ruby on rails, got me into this problem.
Modify gemfile, and add this be:

#below gem 'sqlite3'
gem 'mysql2'

Example database.yaml

development:

  adapter: mysql2

  encoding: utf8

  database: yourdbname

  username: yourusername

  password: "your password"

  host: localhost

  port: 3306