Edit : found what I was doing wrong. Don't try to edit the code directly in the command line (using nano) or it won't work. But you have to open it on the file editor above the command line, by going to db/migrate/ and then add the line there. Same effect but the tutorial doesn't register nano changes.
When I start the exercise I get "Another migration is already named create_messages". That means there is already a file with t.text :content in the 4th line, so the checkbox doesn't turn green and won't let me move forward.
I've forced the migration with "rails generate model Message --force", which means I delete the old file and create a new one, where the line "t.text :content" is not there. I then used nano 20151101114407_create_messages.rb. I've added the fourth line and saved the file but nothing happens. When I use nano again I can see "t.text :content" being added but it still won't check the checkbox for me. Am I missing something or is there actually an issue with the tutorial ?