Increase Elixir version to 1.8

[kevinbader]

• update https://github.com/Accenture/reactive-interaction-gateway/blob/master/version
• check depreciation and update code accordingly
• test the change

[mmacai]

Perhaps we can go directly for 1.9 as it was just released? https://elixir-lang.org/blog/2019/06/24/elixir-v1-9-0-released/?utm_campaign=elixir_radar_196&utm_medium=email&utm_source=RD+Station

• seems like they added built-in release packaging, so we may get rid of Distillery
• change to configuration also looks pretty neat.