Fix CPIO test to not rely on file mode

With different umask when unpacking the archive, the file can have other
mode than expected and this leads to test failure.

This fixes it to expect the same mode as the original file has.

Signed-off-by: Michal Čihař <michal@cihar.com>
Support for running tests in Travis CI

Test against more python versions and lxml as well. The build for
current lxml seems to be broken.

Signed-off-by: Michal Čihař <michal@cihar.com>