Create Travis build infrastructure

[tripleee]

https://travis-ci.org/ provides free CI service for open source projects.

Making use of this resource would basically entail creating a .travis.yml file, and creating an account owned by the libarchive maintainers.

The first half is already done, and included in a rejected pull request. Creating an account and linking to that can obvioulsy only by done by the libarchive maintainers.

So, this ticket asks for you to (1) create a Travis account and (2) then pull in the changes from PR#86.

The part in (2) is useful in its own right if you don't want Travis build integration for yourselves, but obviously, having a centrally managed build saves resources overall.

[kientzle]

See PR #593.