Update Elixir HL7 to newest Version

[Fritzelblitz]

Update elixir_hl7 to 0.9, to allow users of this Library to use newer Versions of the elixir_hl7 Library.

[DarkMarmot]

Seconding this, while you can use override: true to force HL7 to higher versions, it would be good to make this library more flexible as I dont think it cares much.

Perhaps >= 0.8.0 and < 1.0.0?