Bump version to 1.6.6 (#3520)
Bump version to 1.6.6 (#3520)

For release tracking 3519

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
johnbelamaric/coredns,miekg/coredns,yongtang/coredns,johnbelamaric/coredns,chrisohaver/coredns,coredns/coredns,miekg/coredns