This fixes the following assertion:

  Traceback (most recent call last):
    File "neo/master/app.py", line 172, in run
      self._run()
    File "neo/master/app.py", line 182, in _run
      self.playPrimaryRole()
    File "neo/master/app.py", line 302, in playPrimaryRole
      self.backup_app.provideService())
    File "neo/master/backup_app.py", line 114, in provideService
      node, conn = bootstrap.getPrimaryConnection()
    File "neo/lib/bootstrap.py", line 74, in getPrimaryConnection
      poll(1)
    File "neo/lib/event.py", line 160, in poll
      to_process.process()
    File "neo/lib/connection.py", line 504, in process
      self._handlers.handle(self, self._queue.pop(0))
    File "neo/lib/connection.py", line 92, in handle
      self._handle(connection, packet)
    File "neo/lib/connection.py", line 107, in _handle
      pending[0][1].packetReceived(connection, packet)
    File "neo/lib/handler.py", line 125, in packetReceived
      self.dispatch(*args)
    File "neo/lib/handler.py", line 75, in dispatch
      method(conn, *args, **kw)
    File "neo/lib/handler.py", line 159, in notPrimaryMaster
      assert primary != self.app.server
  AttributeError: 'BackupApplication' object has no attribute 'server'

(cherry picked from commit dba07e72)