"An error occurred while compiling Enum: Debug info not available"

When trying to get the example running I get a warning, namely:

```
[warning] Mix compiler :elixir_script was supposed to return :ok | :noop | {:ok | :noop | :error, [diagnostic]} but it returned [%Mix.Task.Compiler.Diagnostic{compiler_name: "elixir_script", details: nil, file: nil, message: "An error occurred while compiling Enum: Debug info not available", position: nil, severity: :error}]
```

Followed by an error:

```
ERROR in ./js/app.js
Module not found: Error: Can't resolve './build/Elixir.Todo.Main.js' in '/home/smoes/git/todo-elixirscript/assets/js'
@ ./js/app.js 22:0-47
```

What am I doing wrong?

```
> iex -v
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]

IEx 1.6.4 (compiled with OTP 19)```