Avoid using serde_json for JSON snapshot serialization

[mitsuhiko]

We might not need serde_json to serialize to JSON snapshots. This seems relatively low priority as someone who uses serde and JSON snapshots probably already has serde_json somewhere else in the project.

One potential benefit however is that we might be able to create a more stable snapshot format that is not affected by changes in serde_json. This is probably less of a concern than with YAML where the format likes to break.

Refs #256

[mitsuhiko]

Fixed by #265