release: up the version to 1.6.8 (#3772)
release: up the version to 1.6.8 (#3772)

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
johnbelamaric/coredns,chrisohaver/coredns,coredns/coredns,miekg/coredns,johnbelamaric/coredns,yongtang/coredns,miekg/coredns