Add Travis-CI config

This will try to run the Saxon transform for each new commit and pull request.
Someone with admin access to the repository will need to configure the hook by following http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in.

Note that [the build currently fails](https://travis-ci.org/nschonni/wcag/builds/19758604) because there are errors in the transform.