chore(package): update eslint-plugin-flowtype to version 3.9.0

Closes #13

Author: greenkeeper[bot]

package.json

@@ -56,7 +56,7 @@
     "eslint": "^5.2.0",
     "eslint-config-airbnb": "^17.0.0",
     "eslint-plugin-babel": "^5.1.0",
-    "eslint-plugin-flowtype": "^2.50.0",
+    "eslint-plugin-flowtype": "^3.9.0",
     "eslint-plugin-import": "^2.13.0",
     "eslint-plugin-jsx-a11y": "^6.1.1",
     "eslint-plugin-react": "^7.10.0",