Elixir 1.4 warnings

[russmatney]

I'm seeing a handful of warnings when compiling on elixir 1.4, so am approaching a PR to clean these up. Hope to get to it in the next few days, but I thought I'd drop a line here to be sure I wasn't recreating any work if anyone else is working on this.

I'm also curious what your thoughts are on including @specs on functions throughout – it helps a ton with maintainability. For example, this line would be much easier to reason about (Keyword.put_new/3, or Map.put_new/3?) if the function had a spec describing the types.

[norpan]

I let all the warnings be when I made a patch, but perhaps it would be good to remove all warnings in the master branch so that we don't have to deal with this every time we make a patch