Fix deprecated time unit warning

[ivan]

Fixes:

warning: deprecated time unit: :microseconds. A time unit should be :second, :millisecond, :microsecond, :nanosecond, or a positive integer
  (elixir) lib/system.ex:898: System.warn/2
  (elixir) lib/system.ex:752: System.convert_time_unit/3
  (elixir) lib/calendar/datetime.ex:280: DateTime.to_unix/2
  (postgrex) lib/postgrex/extensions/timestamptz.ex:33: Postgrex.Extensions.TimestampTZ.encode_elixir/1
  (postgrex) lib/postgrex/type_module.ex:714: Postgrex.DefaultTypes.encode_params/3

[josevalim]

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