Amethyst format is broken for 0.15

0.15 now requires a `List(())` to surround the `ron` file. Example: https://github.com/amethyst/amethyst-starter-2d/pull/14/files#diff-ee91fa974052bd6353c9077e79d522eb

From the changelog for 0.14:

```
Changed SpriteSheetFormat::import_simple to allow importing grid based SpriteSheets (#2023)
Migration Note: Rons need to wrap their content in either Grid() or List()
```

Error:
```
[ERROR][amethyst_assets::storage] "renderer::SpriteSheet": Asset "textures/spritesheet.ron" (handle id: Handle { id: 0 }) could not be loaded: Failed to load asset with name "textures/spritesheet.ron"
[ERROR][amethyst_assets::progress] Error loading handle 0, renderer::SpriteSheet, with name textures/spritesheet.ron: Failed to load asset with name "textures/spritesheet.ron"
caused by: Failed to load asset with name "textures/spritesheet.ron"
Error { inner: Inner { source: Some(Error { inner: Inner { source: Some(Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }), backtrace: None, error: Format("SPRITE_SHEET") } }), backtrace: None, error: Asset("textures/spritesheet.ron") } }
caused by: Format "SPRITE_SHEET" could not load asset
Error { inner: Inner { source: Some(Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }), backtrace: None, error: Format("SPRITE_SHEET") } }
caused by: Failed to parse SpriteSheet
Error { inner: Inner { source: None, backtrace: None, error: LoadSpritesheetError(Parser(ExpectedIdentifier, Position { col: 1, line: 1 })) } }
```

```
$ sheep --version
sheep 0.3.0
```