travis: Only build release binaries for release commits
travis: Only build release binaries for release commits

Building the release binaries added about a minute to the build time,
and is unnecessary for PRs.

ejj/quilt-dev,LuiseV/quilt,kklin/kelda,NetSys/quilt,kklin/quilt,kelda/kelda,kklin/quilt,NetSys/quilt,kayousterhout/quilt,kklin/kelda,quilt/quilt,quilt/quilt,ejj/quilt-dev,NetSys/quilt,ejj/quilt-dev,LuiseV/quilt,kklin/kelda,kelda/kelda,kayousterhout/quilt,ejj/quilt-dev,LuiseV/quilt,quilt/quilt,kayousterhout/quilt,kelda/kelda,kklin/kelda,LuiseV/quilt,quilt/quilt,kklin/quilt,LuiseV/quilt,kklin/quilt,NetSys/quilt,NetSys/quilt,kayousterhout/quilt,kelda/kelda