ci: Add kube v1.18.5 to E2E testing
ci: Add kube v1.18.5 to E2E testing

Updated travis CI to test cephcsi with
kubernetes v1.18.5

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi