build: add TravisCI configuration for drive repo.
build: add TravisCI configuration for drive repo.

Change-Id: Idcaaa9c117ba265e894fb99478b54c0e906b870f
Reviewed-on: https://upspin-review.googlesource.com/16540
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/drive