chore(Elixir): fix warnings emitted from v1.17.0

[tomciopp]

@voltone Let me know if you have any questions.

[voltone]

Thanks for your PR! I just merged #73 and it formats the code using a more recent formatter that uses charlist sigils. I believe that supersedes this PR, am I right?

[tomciopp]

@voltone That PR handles most of the issues, but there are still some charlists in doctests. I can push another PR that just handles those.

[tomciopp]

@voltone There is also a warning thrown by the deprecated, System.get_pid/0 function. However I have left that out of the new PR as it looks like the replacement System.pid has only been around since Elixir version 1.9.0.

[voltone]

@voltone There is also a warning thrown by the deprecated, System.get_pid/0 function. However I have left that out of the new PR as it looks like the replacement System.pid has only been around since Elixir version 1.9.0.

I know, and since it is only used in tests I don't see much reason to drop support for Elixir < 1.9 just for that...

[voltone]

Superseded by #73 and #74