Add "asset" section to BoundsChecking test models

These test assets were trimmed down and erroneously didn't have an
"asset" section or version number, which is required by the glTF spec.

This fixes test failures with pull request #162.

models/BoundsChecking/invalid-buffer-index.gltf

@@ -46,5 +46,8 @@
       "max": [2],
       "min": [0]
     }
-  ]
+  ],
+  "asset": {
+    "version": "2.0"
+  }
 }

models/BoundsChecking/invalid-buffer-view-index.gltf

@@ -29,5 +29,8 @@
       "max": [2],
       "min": [0]
     }
-  ]
+  ],
+  "asset": {
+    "version": "2.0"
+  }
 }

models/BoundsChecking/invalid-primitive-indices.gltf

@@ -29,5 +29,8 @@
       "max": [2],
       "min": [0]
     }
-  ]
+  ],
+  "asset": {
+    "version": "2.0"
+  }
 }