relax elixir version to allow 0.13.0 to be used as well

Not sure what version pogo is aiming to target, so went with `~> 0.12` to be safe.

Considered going with `~> 0.13.0`, but that would lock out existing users on ~ 0.12.0 still.

Also considered `~> 0.12.0 or ~> 0.13.0`, but that just doesn't make as much sense as just relaxing it.