Updates for mix and elixir

Hi! Here are some things that I had to do to get this running on my (bleeding edge 0.10.1-dev) elixir app.

Some explanation:

Ran into an issue where installing exjson via mix deps wasn't generating  `exjson.app` in ebin. Adding in a mix configuration fixes that, but honestly I'm not sure why (heh).

Also, the API for inspect was changed up a bit in 0.10.0+, so there's a quick fix here as well.