Convert CalliopeException to struct

This Pull Request rewrites the CalliopeException as a struct which will be the way to do it in Elixir 0.13.3

I have also added a .travis.yml file that runs the tests against Elixir master.

[![Build Status](https://travis-ci.org/vicmargar/calliope.svg?branch=exception-record)](https://travis-ci.org/vicmargar/calliope)

I don't know if you can use this as it is since this won't work until Elixir 0.13.3 is released but at least you can take a look at it and let me know!