travis: drop go1.8
travis: drop go1.8

with the update to `golang.org/x/crypto`, the commit
https://github.com/golang/crypto/commit/c7dcf104e3a7a1417abc0230cb0d5240d764159d
depends on a stdlib package introduced in go1.9

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

vbatts/go-mtree,vbatts/go-mtree