use Travis CI for testing

The kriskowal/zip#8 pull request is currently marked with a message that says “The Travis build failed”.

I do not know the exact reason for that, but I suspect that the missing `.travis.yml` configuration is the cause.

Here I contribute a set of changes that allows using [Travis CI](https://travis-ci.org/) as a hosted continuous integration service for your project, according to [their docs](http://about.travis-ci.org/docs/).