It's expected that pgid to be setup for "foreground" running server…

… rather than for "background".

So it should be changed to `set_pgid if foreground?`

See discussion [here](rails@e16e085#r18804964)

lib/spring/server.rb

@@ -36,7 +36,7 @@ def boot
       Spring.verify_environment
 
       write_pidfile
-      set_pgid unless foreground?
+      set_pgid if foreground?
       ignore_signals unless foreground?
       set_exit_hook
       set_process_title