fixed immutable.js to be 3.7.x because 3.8.0 breaks flow checks

Summary: Closes #7014

Differential Revision: D3188876

fb-gh-sync-id: 800e1bf31dfd698a4aa076e64bf27b090b841e40
fbshipit-source-id: 800e1bf31dfd698a4aa076e64bf27b090b841e40

Author: bestander

package.json

@@ -148,7 +148,7 @@
     "fbjs-scripts": "^0.4.0",
     "graceful-fs": "^4.1.2",
     "image-size": "^0.3.5",
-    "immutable": "^3.7.5",
+    "immutable": "~3.7.6",
     "joi": "^6.6.1",
     "json-stable-stringify": "^1.0.1",
     "json5": "^0.4.0",