Add missing dep

Fixes #4399

Author: mjackson

packages/react-router/package.json

@@ -39,6 +39,7 @@
   },
   "dependencies": {
     "history": "^4.5.1",
+    "invariant": "^2.2.2",
     "loose-envify": "^1.3.1",
     "path-to-regexp": "^1.5.3",
     "warning": "^3.0.0"