Update scratch-paint to fix the undo bug

package.json

@@ -91,7 +91,7 @@
     "scratch-audio": "0.1.0-prerelease.1523977528",
     "scratch-blocks": "0.1.0-prerelease.1525125321",
     "scratch-l10n": "2.0.20180108132626",
-    "scratch-paint": "0.2.0-prerelease.20180503182655",
+    "scratch-paint": "0.2.0-prerelease.20180504180528",
     "scratch-render": "0.1.0-prerelease.20180502115145",
     "scratch-svg-renderer": "0.1.0-prerelease.20180423193917",
     "scratch-storage": "0.4.1",