bump immer to v6 (#407)

Author: markerikson

package-lock.json

@@ -58,7 +58,7 @@
     "src"
   ],
   "dependencies": {
-    "immer": "^6.0.0-alpha.4",
+    "immer": "^6.0.0",
     "redux": "^4.0.0",
     "redux-thunk": "^2.3.0",
     "reselect": "^4.0.0"