Adopt newer syntax

Author: YuKitAs

app/controllers/authentication_controller.rb

@@ -4,7 +4,7 @@ class AuthenticationController < ApplicationController
   def login
     command = AuthenticateUser.call(params[:email], params[:password])
 
-    if command.success?
+    if command.successful?
       render(json: { auth_token: command.result })
     else
       render(json: { error: command.errors }, status: :unauthorized)