scylla-bench: update go to 1.13
scylla-bench: update go to 1.13

Currently we used go 1.9.2 for installing scylla-bench, but it doesn't work now.
Update go version to solve the problem.

Command: 'go get github.com/scylladb/scylla-bench'
Exit code: 2
Stderr:
  # github.com/gocql/gocql
  go/src/github.com/gocql/gocql/control.go:156:7: randr.Shuffle undefined (type *"math/rand".Rand has no field or method Shuffle)
  go/src/github.com/gocql/gocql/policies.go:352:6: rand.Shuffle undefined (type *"math/rand".Rand has no field or method Shuffle)
  go/src/github.com/gocql/gocql/policies.go:865:4: r.Shuffle undefined (type *"math/rand".Rand has no field or method Shuffle)

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests