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
TemporaryJobs: allow use without @Rule

Make before() and after() public so TemporaryJobs setup and teardown
can be manually performed.
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.
Merge pull request #128 from spotify/rohan/circleci

tests: Only pull busybox if it not found locally
Merge pull request #129 from spotify/dano/temporary-jobs-public-before-after

TemporaryJobs: allow use without @Rule
cli: inspect: human readable volume output
agent: poll instead of waiting for container exit

We're doing this poll loop instead of docker.waitContainer() because we
saw the agent hang forever on waitContainer after the socket got into a
weird half-open state where the kernel (netstat/lsof) would only show
one end of the connection and restarting docker would not close the
socket.

¯\_(ツ)_/¯
remove unused class