[Dev Deps] add missing dev dep

Author: ljharb

package.json

@@ -44,6 +44,7 @@
     "eslint-config-airbnb-base": "^13.0.0",
     "eslint-plugin-flowtype": "^2.49.3",
     "eslint-plugin-import": "^2.13.0",
+    "estraverse": "^4.2.0",
     "expect": "^21.2.1",
     "flow-bin": "^0.66.0",
     "in-publish": "^2.0.0",