travis: move `cgroup-v2` out of `allow_failures`
travis: move `cgroup-v2` out of `allow_failures`

`cgroup-v2` was marked `allow_failures` because of the flakiness of VirtualBox VM: dc7d0bf

The flakiness seems to have gone away since we switched from VirtualBox to QEMU/KVM and increased HW resources: b8eed86

Close #2301

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

rhatdan/runc,opencontainers/runc,xiaochenshen/runc,mikebrow/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,dqminh/runc,dqminh/runc,estesp/runc,mikebrow/runc,xiaochenshen/runc,opencontainers/runc,rhatdan/runc,xiaochenshen/runc,estesp/runc,kolyshkin/runc,rhatdan/runc,dqminh/runc,dqminh/runc,kolyshkin/runc,opencontainers/runc,mikebrow/runc,estesp/runc,opencontainers/runc