Lotsa Warnings ... ⚠️> Update Phoenix/Elixir Dependencies

When running (compiling) the project on `localhost` using `Elixir 1.8.1` we see _many_ warnings ...

_Most_ (_if not all_) of these are due to the underlying libraries `Phoenix`, `Ecto`, `Poison`, etc. being out-of-date. So we need to run [`mix deps.update`](https://tosbourn.com/keeping-elixir-packages-updated/) and fix any broken tests.

# Todo
+ [ ] Update all Elixir/Phoenix packages to latest versions
+ [ ] Fix any broken tests
+ [ ] ideally we should have ` --warnings-as-errors` enabled in a project that is kept up-to-date.
That way _anyone_ working on the project will _immediately_ see when a warning appears.
It was removed in https://github.com/dwyl/library/commit/f38b2c4e43d094943182be4a89c72406994019d0 to make Travis-CI pass, but I feel it should be re-instated.