Speeding up Elixir project build times on Travis CI

[Nelson]

At present our Elixir section does not advise people to cache dependencies/build on Travis-CI to speed up build time: https://github.com/dwyl/learn-travis#elixir-lang-project

Add the following lines to .travis.yml file:

cache:
  directories:
    - _build
    - deps

Thanks to @bcardarella of Dockyard for this insight:
https://dockyard.com/blog/2015/12/02/speeding-up-elixir-project-build-times-on-travis-ci