build: update TravisCI to use Go 1.9
build: update TravisCI to use Go 1.9

Change-Id: I6396d5bfb00a9cf84b2fff8269c9a25d2438621f
Reviewed-on: https://upspin-review.googlesource.com/13482
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/aws