Add omniauthable. Fixes #18

app/models/user.rb

@@ -1,8 +1,7 @@
 class User < ApplicationRecord
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable and :omniauthable
-  devise :masqueradable, :database_authenticatable, :registerable,
-         :recoverable, :rememberable, :trackable, :validatable
+  devise :masqueradable, :database_authenticatable, :registerable, :recoverable, :rememberable, :trackable, :validatable, :omniauthable
 
   validates :first_name, :last_name, presence: true