travis: Add a travis.yml for doing CI after commits

Instruct travis-ci to build systemd and create a tarball. In case
of an error travis-ci will complain on IRC. The systemd testsuite
currently requires the host to have a recent version of systemd
installed and running. This is not the case for the Ubuntu VM of
travis-ci. This means make check and make distcheck will result in
a build failure and to avoid this these commands are not executed.