Target node 6.9.0

Target the previous Node LTS instead of "whatever version of Node Adam
is running today." 🤦🏻‍♂️

Author: adamwathan

package.json

@@ -53,13 +53,16 @@
         "env",
         {
           "targets": {
-            "node": "current"
+            "node": "6.9.0"
           }
         }
       ],
       "stage-2",
       "stage-3",
       "react"
     ]
+  },
+  "engines": {
+    "node": ">=6.9.0"
   }
 }