travis: use Go 1.9.4 with TARGET_GO_VERSION
travis: use Go 1.9.4 with TARGET_GO_VERSION

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

etcd-io/etcd,mqliang/etcd,sinsharat/etcd,mqliang/etcd,xiang90/etcd,mkumatag/etcd,vimalk78/etcd,rnaveiras/etcd,etcd-io/etcd,coreos/etcd,sebrandon1/etcd,dannysauer/etcd,bdarnell/etcd,ajityagaty/etcd,mqliang/etcd,cockroachdb/etcd,glevand/coreos--etcd,vimalk78/etcd,ajityagaty/etcd,lorneli/etcd,dannysauer/etcd,whuwxl/etcd,rnaveiras/etcd,cockroachdb/etcd,coreos/etcd,sinsharat/etcd,joshix/etcd,glevand/coreos--etcd,etcd-io/etcd,joshix/etcd,gyuho/etcd,ajityagaty/etcd,lorneli/etcd,li-ang/etcd,whuwxl/etcd,xiang90/etcd,fanminshi/etcd,gyuho/etcd,mkumatag/etcd,bdarnell/etcd,sebrandon1/etcd,li-ang/etcd,mkumatag/etcd,fanminshi/etcd,rnaveiras/etcd