rewrite erlang-generated log level from warn -> warning

[dch]

This might not be the best way to handle this error but it's very convenient.

I often use 3rd party dependencies in the erlang ecosystem, that emit
:warn instead of :warning levels, which creates a duplicate notification
from Logger every time. This is very annoying:

warning: the log level :warn is deprecated, use :warning instead
  (logger 1.18.4) lib/logger.ex:1210: Logger.elixir_level_to_erlang_level/1
  (logger 1.18.4) lib/logger.ex:660: Logger.compare_levels/2
  (logger_syslog_backend 1.0.3) lib/logger_syslog_backend.ex:22: LoggerSyslogBackend.handle_event/2
  (stdlib 6.2.2) gen_event.erl:1921: :gen_event.server_update/4
  (stdlib 6.2.2) gen_event.erl:1903: :gen_event.server_notify/4
  (stdlib 6.2.2) gen_event.erl:1640: :gen_event.handle_msg/6
  (stdlib 6.2.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

etc.

If this is a suitable fix, please tag 1.0.4 as well afterwards! thanks 🍈 🥭 🥝