Postgres 9.4, replication slots, doesn't work failover master
You cannot just jump back to the old master, as it is said here: 


  Once failover to the standby occurs, there is only a single server in
  operation. This is known as a degenerate state. The former standby is
  now the primary, but the former primary is down and might stay down.
  To return to normal operation, a standby server must be recreated,
  either on the former primary system when it comes up, or on a third,
  possibly new, system.


For this, you need a recovery.conf on the old master.  This should be nearly the same as the present one, only the connection has to point elsewhere.