Build with Go 1.11

Build upspin with Go 1.11 which is the most recent release.

Change-Id: Ifa3a69dedbbcaad18ed24dc27133f0acce490623
Reviewed-on: https://upspin-review.googlesource.com/c/19200
Reviewed-by: adg@google.com <adg@google.com>

.travis.yml

@@ -3,7 +3,7 @@ os:
 - osx
 language: go
 go:
-- 1.9.x
+- 1.11.x
 go_import_path: upspin.io
 install: true
 script: go test -short -race ./...