Point at vision extension feature branch for VM

package.json

@@ -94,7 +94,7 @@
     "scratch-paint": "0.2.0-prerelease.20180222192821",
     "scratch-render": "0.1.0-prerelease.1516837442",
     "scratch-storage": "0.4.0",
-    "scratch-vm": "0.1.0-prerelease.1518812805-prerelease.1518812820",
+    "scratch-vm": "github:thisandagain/scratch-vm#feature/vision",
     "selenium-webdriver": "3.6.0",
     "startaudiocontext": "1.2.1",
     "style-loader": "^0.20.0",