Fix doctests

[mmmries]

Fixes #39

Also updates the erlang/elixir versions that we run on travis and ignores the mix.lock from the git repo.

/cc @jazzyb @obmarg

[obmarg]

What's the thinking behind ignoring mix.lock? Always thought that was meant to be in the git repository. To quote the getting started guide:

You will notice that when you add a dependency to your project, Mix generates a mix.lock file that guarantees repeatable builds. The lock file must be checked in to your version control system, to guarantee that everyone who uses the project will use the same dependency versions as you.

[mmmries]

@obmarg the reasoning is so that travis will always build with the latest version of each of our dependencies (within the constraints that we declare in mix.exs). I started ignoring my Gemfile.lock files when I publish ruby gems a while ago because it helped me see breakages between my code and other dependent libraries before I released updates.

My rule of thumb is that I usually include lock files in git repos for my applications, but I ignore them in my published libraries because the users of my library will always resolve their own dependencies so I only want to rely on the constraints that I define in my library.

[obmarg]

Fair enough. Looks good to me then.