Working with Ruby 2.3.1 and Rails 5.1.1
You will also need to install the twitter gem
gem install twitter
Install the database with the following command :
bin/rails db:migrate
You need to first set the configuration for the twitter API (to generate at https://apps.twitter.com). You will need Read, write and direct messages permission.
bin/rails twitter:set_config consumer_key consumer_secret access_token access_token_secret
You also need to configure the message that you want to send to your new followers. You can use the variable #NAME that will be replaced with the screen name of the follower.
bin/rails settings:set_message "Hello #NAME, this is a message for my new followers."
Then, load the list of your current followers, to compare with the future list and send DM to your new followers.
bin/rails twitter:init_list
Finally, you can set a cron job or manually send DM to your new followers with the following command :
bin/rails twitter:send_dm