Disable parallel jobs for the osx (due to CPU time deficit) in travis-ci

Disable parallel jobs for the osx (due to CPU time deficit) in travis-ci

As you can see right now linux workers has zero failed tests, while osx
workers has 18 failed tests:
  [bufferevent_connect_hostname_emfile FAILED]
  [bufferevent_pair_release_lock FAILED]
  [bufferevent_timeout FAILED]
  [bufferevent_timeout_filter FAILED]
  [bufferevent_timeout_pair FAILED]
  [common_timeout FAILED]
  [del_wait FAILED]
  [immediatesignal FAILED]
  [loopexit FAILED]
  [loopexit_multiple FAILED]
  [monotonic_res FAILED]
  [no_events FAILED]
  [persistent_active_timeout FAILED]
  [persistent_timeout_jump FAILED]
  [signal_switchbase FAILED]
  [signal_while_processing FAILED]
  [simpletimeout FAILED]
  [usleep FAILED]

And this patch should remove from this list time related failures
(though maybe not all of them).