:warn -> :warning

[ryanlchandler]

Newer versions of Elixir are aligned with Erlang log level names but to maintain backwards compatibility :warn is still being passed to the backend. However, the backend is calling Logger.compare_levels which in turn logs the warning below. This PR converts :warn to :warning before passing the levels to Logger.compare_levels and avoids littering your log files with these messages.

warning: the log level :warn is deprecated, use :warning instead
  (logger 1.16.3) lib/logger.ex:1168: Logger.elixir_level_to_erlang_level/1
  (logger 1.16.3) lib/logger.ex:627: Logger.compare_levels/2
  (airbrakex 0.1.9) lib/airbrakex/logger_backend.ex:63: Airbrakex.LoggerBackend.meet_level?/2
  (airbrakex 0.1.9) lib/airbrakex/logger_backend.ex:30: Airbrakex.LoggerBackend.handle_event/2
  (stdlib 5.2) gen_event.erl:814: :gen_event.server_update/4
  (stdlib 5.2) gen_event.erl:796: :gen_event.server_notify/4
  (stdlib 5.2) gen_event.erl:538: :gen_event.handle_msg/6
  (stdlib 5.2) proc_lib.erl:241: :proc_lib.init_p_do_apply/3