build: update TravisCI to use Go 1.9

Change-Id: I02f2894111bb6c21aad094bae0d0b407f27f576e
Reviewed-on: https://upspin-review.googlesource.com/13480
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/.travis.yml b/.travis.yml
index 6b8dc5b..8bb7fc0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 language: go
 go:
-- 1.8.x
+- 1.9.x
 go_import_path: exp.upspin.io
 script: go test -short -race ./...
 notifications: