build: bump travis build go version to 1.13

Change-Id: Ic0b90997bb9137b0407d1cea64b4667ea944ce43
Reviewed-on: https://upspin-review.googlesource.com/c/gcp/+/19387
Reviewed-by: Eric Grosse <grosse@gmail.com>
diff --git a/.travis.yml b/.travis.yml
index 029907a..ba81720 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 language: go
 go:
-- 1.9.x
+- 1.13.x
 go_import_path: gcp.upspin.io
 install: true
 script: go test -short -race ./...