Travis CI: Only build tagged versions

We only want to use Travis CI for releases. Everything else will get built
by CircleCI.
Merge pull request #127 from spotify/rohan/travis

Travis CI: Only build tagged versions
tests: Only pull busybox if it not found locally

We docker pull busybox before each test, but this can be slow in a shared
build environment. As a workaround, only pull if the image doesn't exist
locally.