Test: travis: refrain from relying on (and bothering) relaxng.org host

We will cache relaxng.rng file using the respective Travis CI facility.
Note that we really don't need to ship that file with pacemaker itself,
plus it would be one more license to care about if we borrowed that
file from BSD-covered jing-trang project:
https://github.com/relaxng/jing-trang/blob/master/eg/relaxng.rng
So we rather stay sole user of that file than redistributor...