The current Travis setup scripts copy in rkt, but do not set up a
default container network.
Here we copy the container network setup over from the vagrant setup
scripts.
The changes introduces in #3256 require at least rkt 1.27.0 because of
a bug in the JSON output of `rkt status` in previous versions.
Here we upgrade all references to rkt's minimum version, and also make
travis and vagrant use this version when running tests.
Finally we add a CHANGELOG notice.