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

Change-Id: Id4e9555387663b289d5cf6c9779bebe62e90e995
Reviewed-on: https://upspin-review.googlesource.com/13483
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin