chore(CI): add travis.yml for Travis CI

Travis CI has been added using a `.travis.yml` file. This sets up Elixir
in a before_install script. It is based on the one provided by the
Phoenix web framework:

 > https://github.com/phoenixframework/phoenix/blob/ \
 8cffae4744e30e06e6770b4170e2884edd09a85f/.travis.yml
docs(CONTRIBUTING.md): add a note about subject tense and case

The contribution guidelines previously did not mention that the commit
messages should be lowercase and in the past tense. This has been
updated with an example.