Merge pull request mozilla#344 from mozilla/greenkeeper-flow-bin-0.28.0

Update flow-bin to version 0.28.0 🚀

Author: kumar303

package.json

@@ -77,7 +77,7 @@
     "coveralls": "2.11.9",
     "deepcopy": "0.6.3",
     "eslint": "3.0.1",
-    "flow-bin": "0.27.0",
+    "flow-bin": "0.28.0",
     "grunt": "1.0.1",
     "grunt-contrib-clean": "1.0.0",
     "grunt-contrib-copy": "1.0.0",