Changed json import to work with texturepacker 3.7.1 json array

[Patrikgrinsvall]

This commit was needed because the format of the latest version of TexturePacker seems to have a different format than the plugin wants.

The way i create Tilemaps in texture packer in order to get this commit to work is.

1. Create new project, JSON (Array).
2. Add sprites and select all images
3. Enter a name in Data file in advanced in Data tab.
4. Publish