Fix generator to correctly inject content into the user model in rails 5

[ethangk]

I ran rails g model user in a fresh rails 5 api project, then the install command. Below is the resulting contents of the user model

class User < ApplicationRecord
  # Include default devise modules.
  devise :database_authenticatable, :registerable,
          :recoverable, :rememberable, :trackable, :validatable,
          :confirmable, :omniauthable
  include DeviseTokenAuth::Concerns::User
end

[ethangk]

This is to fix #627

[booleanbetrayal]

thanks for this @ethangk !

[ethangk]

My pleasure!