Add simple .travis.yml.
Add simple .travis.yml.

It simply runs gazel --validate and then bazel build //... . When we
add testing, just add bazel test //... to the script section.

ixdy/kubernetes-repo-infra,mikedanese/gazel,spxtr/gazel,ixdy/kubernetes-repo-infra,kubernetes/repo-infra,kubernetes/repo-infra,kubernetes/repo-infra,ixdy/kubernetes-repo-infra,spxtr/gazel,mikedanese/gazel