Test multiple versions of Elixir in CI; fix incompatibilities with Elixir v1.11

This merge request modifies the configuration of IBDNS’s continuous integration so that it ensures that the code stays compatible with a wide range of Elixir releases.

Following some investigation, the range of Elixir versions currently supported is 1.11 through 1.16. Elixir 1.17 passes unit tests, but false warnings from Dialyzer prevent us from using that version in CI until a bugfix release is made of Elixir 1.17.

Some code was rewritten so as to avoid using operators or constructs not available in older versions of Elixir.

This PR fixes issue #45 (closed).