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

[nelsonic]

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 being out-of-date.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 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 f38b2c4 to make Travis-CI pass, but I feel it should be re-instated.

[nelsonic]

Pre-commit (credo) fails on master:

Pre-commit hook is not documented ... 😞 #87

[nelsonic]

@iteles given that this project is not in active use (yet!) I've downgraded the priority of this issue to P2.
(I'm busy trying to find/filter the P1's and see what needs to be done next ...)