travis: use go1.{6,7,8} and tip
travis: use go1.{6,7,8} and tip

The ancient go 1.5 has been removed.

Signed-off-by: Valentin Rothberg <6db4ac3d4f2c7b0d4923f1257f87bfb6a65f568b@suse.com>

SUSE/zypper-docker,SUSE/zypper-docker,SUSE/zypper-docker