travis: upgrade golang to 1.11.1
travis: upgrade golang to 1.11.1

Since Go 1.11, `go vet` checks format strings in Printf wrappers
a little stricter than previous versions. [1] Also `go test` started
to run `go vet` automatically. As a result, with Go 1.11 or newer,
running `go test` causes `go vet` to fail in some cases.

That's why several format strings bugs had not been discovered until
https://github.com/containers/image/pull/522. Travis CI did not catch
the bugs, because the container image for travis CI is built based on
Go 1.9.

To avoid such cases in the future, we should upgrade Golang to 1.11.1.

[1] https://golang.org/doc/go1.11#vet

Signed-off-by: Dongsu Park <696fb7639d678a1e692fa501217d7509545d7331@kinvolk.io>

containers/image,containers/image