Bump scratch-blocks

Bumps [scratch-blocks](https://github.com/LLK/scratch-blocks) from 0.1.0-prerelease.20200416142646 to 0.1.0-prerelease.20200421135214.
- [Release notes](https://github.com/LLK/scratch-blocks/releases)
- [Commits](https://github.com/LLK/scratch-blocks/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package-lock.json

@@ -108,7 +108,7 @@
     "redux-throttle": "0.1.1",
     "rimraf": "^2.6.1",
     "scratch-audio": "0.1.0-prerelease.20190925183642",
-    "scratch-blocks": "0.1.0-prerelease.20200416142646",
+    "scratch-blocks": "0.1.0-prerelease.20200421135214",
     "scratch-l10n": "3.8.20200421213407",
     "scratch-paint": "0.2.0-prerelease.20200213174123",
     "scratch-render": "0.1.0-prerelease.20200422235043",