Fix centos version to 7.*

[resouer]

Fix: #159
h8s should work on CentOS 7 or newer.

[feiskyer]

@resouer Something is wrong with travis CI:

Exiting with status 1
!!! Error in ./hack/verify-codegen.sh:26
  '"${KUBE_ROOT}/hack/update-codegen.sh" --verify-only' exited with status 1
Call stack:
  1: ./hack/verify-codegen.sh:26 main(...)
Exiting with status 1

[resouer]

@feiskyer It's because 2017 is not supported in boilerplate, it should be fixed in #158