Fix SystemStackError when running deploy:migrate (#247)

Author: mattbrictson

lib/capistrano/tasks/migrations.rake

@@ -36,6 +36,6 @@ namespace :load do
     set :conditionally_migrate, fetch(:conditionally_migrate, false)
     set :migration_role, fetch(:migration_role, :db)
     set :migration_servers, -> { primary(fetch(:migration_role)) }
-    set :migration_command, -> { fetch(:migration_command, 'db:migrate') }
+    set :migration_command, fetch(:migration_command, 'db:migrate')
   end
 end