Use Omniauthable user model with admin role

app/controllers/admin_controller.rb

@@ -1,3 +1,8 @@
 class AdminController < ApplicationController
-  before_action :authenticate_admin!
+  before_action :authenticate!
+
+  def authenticate!
+    authenticate_user!
+    raise SecurityError unless current_user.admin?
+  end
 end