Remove unused sync-exec

package.json

@@ -31,7 +31,6 @@
     "react-sticky": "^2.1.0",
     "reflux": "^0.2.8",
     "selectize": "^0.12.1",
-    "sync-exec": "^0.5.0",
     "webpack": "^1.9.10"
   },
   "private": true,