build: add TravisCI configuration for b2 repository
build: add TravisCI configuration for b2 repository

Update #273

Change-Id: I9f639f0196616e5e329c9e73371f2a52bd63cf1f
Reviewed-on: https://upspin-review.googlesource.com/13520
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/b2