Add Travis CI config

I don't know if this is something wanted, but here's my PR to add Travis CI yaml file.

It will run using Elixir 0.14.1, 0.14.2 and 0.14.3: https://travis-ci.org/edgurgel/amqp/builds/30956261

You just need to enable the webhook: http://docs.travis-ci.com/user/getting-started/#Step-two%3A-Activate-GitHub-Webhook