Upgrade fsevents to 1.1.2 (#2550)

This eliminates a `yarn` warning on OSX machines running Node.js 8. See
here for details: fsevents/fsevents#170

You can see an example of the warning in the second screenshot in
facebook/create-react-app#2422 (comment)

package.json

@@ -63,6 +63,6 @@
     "react-dom": "^15.5.4"
   },
   "optionalDependencies": {
-    "fsevents": "1.0.17"
+    "fsevents": "1.1.2"
   }
 }