All tests passing causes annotate to fail to build

If all tests pass, this occurs:
```

2017-07-10 18:00:17 INFO Reading annotation body from STDIN
--
  | 2017-07-10 18:00:18 FATAL Failed to annotate build: POST https://agent.buildkite.com/v3/jobs/00cfa11a-e0fc-42b5-94fc-83d42aaff7d1/annotations: 400 An annotation body is required
```

The reason why is `junit.rb` exists if `all_failures` is empty, and doesn't write to STDOUT.

A good fix would be to change `junit.sh` to check if `annotation.md` has any text, and then show either a success annotation with a message like "All tests pass", otherwise show the error annotation. That would look like this:

```echo "--- :buildkite: Creating annotation"
if [ `cat tmp/annotation.md | wc -l` -eq 0 ] ; then
buildkite-agent annotate --context junit --style success "All tests pass! :+1:"
else
buildkite-agent annotate --context junit --style error < tmp/annotation.md
fi
```

Start with junit.sh and the annotation output produced by junit.rb in tmp/annotation.md. Run the pipeline with all tests passing and inspect the buildkite-agent annotation behavior, then verify that a successful annotation appears for an empty file while failure output still produces the error annotation.

Written by the indexing model from the issue text.