Updating Phoenix and Elixir

[davejlong]

Cog currently runs on Elixir 1.3.2 and Phoenix 1.2. With Elixir 1.5 and Phoenix 1.3 now generally available we should look into upgrade paths. for the platform. There are at least some dependency issues with upgrading to Elixir 1.5 and Erlang 20. The emqttd and lager module's, for example, do not seem to run on newer Elixir/Erlang versions.