Add docs badge

Hi there,

I saw you tried to add this project to Inch CI. As mentioned in the post over at Phoenix, I do not yet have a good workflow for Elixir projects. 

This PR would add `inch_ex` as a dependency to your project and report the docs to Inch CI via Travis (and build the badge for you).

If you have any questions or remarks, just post them here or in the [Phoenix discussion](https://github.com/phoenixframework/phoenix/pull/404). :neckbeard: