build: add TravisCI configuration file
build: add TravisCI configuration file

Change-Id: I1e649f3d4ca7573385759bc2d92332dbde67533a
Reviewed-on: https://upspin-review.googlesource.com/8960
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp