Use proper name for add_migrations

Running the installer on a fresh master (v2.11) Solidus
the migrations are not copied in the Rails application.

We need to use the proper engine name in the FROM parameter.

This commit fixes that.

Author: peterberkenbosch

lib/generators/solidus/auth/install/install_generator.rb

@@ -18,7 +18,7 @@ def generate_devise_key
         end
 
         def add_migrations
-          run 'bundle exec rake railties:install:migrations FROM=solidus_auth_devise'
+          run 'bundle exec rake railties:install:migrations FROM=solidus_auth'
         end
 
         def run_migrations