Remove elixir 1.4 deprecations

[clekstro]

Elixir 1.4 deprecates the use of defoverridable with private functions and recommends using Enum.uniq_by/2 instead of Enum.uniq/2.

This was adding a fair bit of noise to my test suite after upgrading, so assuming this is merged it might be good to get a new version released so others don't run into it.

[coveralls]

Coverage remained the same at 93.735% when pulling 9912277 on clekstro:remove-elixir-1.4-deprecations into 2663050 on parroty:master.

[coveralls]

Coverage remained the same at 93.735% when pulling 6c7419c on clekstro:remove-elixir-1.4-deprecations into 2663050 on parroty:master.

[parroty]

Thanks!