Fix simple example dependencies

Author: djhi

examples/simple/package.json

@@ -31,10 +31,10 @@
         "ra-input-rich-text": "^2.0.0",
         "ra-language-english": "^2.0.0",
         "ra-language-french": "^2.0.0",
-        "ra-tree-core": "^1.0.0",
-        "ra-tree-ui-materialui": "^1.0.0",
-        "ra-tree-language-english": "^1.0.0",
-        "ra-tree-language-french": "^1.0.0",
+        "ra-tree-core": "^2.0.0",
+        "ra-tree-ui-materialui": "^2.0.0",
+        "ra-tree-language-english": "^2.0.0",
+        "ra-tree-language-french": "^2.0.0",
         "react": "~16.3.1",
         "react-admin": "^2.0.0",
         "react-dom": "~16.3.1",