Enable Travis CI
Created by: undu
The results of this PR's build can be seen in https://travis-ci.com/undu/udocker
I don't know how useful can this testing be for the development, and the travis environment probably needs to be tweaked in order to get better results.
I've also added a gitignore so files created by testing to avoid getting them accidentally added to the repo and added a linting step since it is part of the contributing guidelines.
To actually enable Travis, someone in indigo-dc must visit https://travis-ci.com and enable CI for this repo.