This commit fixes an upstream issue with babel and node causing webpack

not to be able to run

nodejs/node#32852

This also updates Mirador to the latest RC

package.json

@@ -3,11 +3,11 @@
   "private": true,
   "dependencies": {
     "@babel/core": "^7.7.5",
-    "@babel/preset-env": "^7.7.6",
+    "@babel/preset-env": "^7.9.0",
     "@babel/preset-react": "^7.7.4",
     "@rails/webpacker": "^4.2.0",
     "core-js": "^3.4.8",
-    "mirador": "^3.0.0-beta.10",
+    "mirador": "^3.0.0-rc.4",
     "prop-types": "^15.7.2",
     "react": "^16.12.0",
     "react-dom": "^16.12.0",