Follow these instructions to configure Unity Editor (v4.3 and up) :

1. Enable External option in Unity → Preferences → Packages → Repository. (Skip this step in v4.5 and up)
2. Switch to Visible Meta Files in Edit → Project Settings → Editor → Version Control Mode.
3. Switch to Force Text in Edit → Project Settings → Editor → Asset Serialization Mode.
4. Save the scene and project from File menu.