fix(package): update core-decorators to version 0.20.0

Author: greenkeeper[bot]

package.json

@@ -67,7 +67,7 @@
     "ampersand-events": "^2.0.2",
     "browser-saveas": "^1.0.1",
     "classnames": "^2.2.5",
-    "core-decorators": "^0.19.0",
+    "core-decorators": "^0.20.0",
     "custom-event-polyfill": "^0.3.0",
     "highlight.js": "^9.12.0",
     "howler": "^2.0.2",