Travis fixes

Here's a step towards things I mentioned in https://github.com/rmies/monad/issues/1

It's not using the latest Elixir yet - there might be you don't use it yet on your machines - but rather making it working on travis while retaining your versions & deps.

A little summary:
* It makes no sense to say in mix.exs that a specific Elixir version is required and then pull a recent unstable stuff from git. A good solution is to use exact Elixir ver as specified in mix.exs - they are also available in precompiled form which speeds up travis builds from minutes to seconds, usually.
* ex_doc is definitely not needed on travis and I believe it's not needed when using prod env as well