YCM client now sends utf-8 encoded JSON.

Previously, we'd just use json.dumps() to dump out JSON. By default,
ensure_ascii is set to true and non-ASCII chars are encoded as \uXXXX.

Problems seem to happen with other text in the data then not being utf8. I'm not
sure why, still can't repro. This should go away now that we explicitly build a
unicode string which we then encode as utf8.

Hopefully fixes #821.