Lock Babel version until babel/babel#1972 is fixed

Author: gaearon

package.json

@@ -36,7 +36,7 @@
   "homepage": "https://github.com/gaearon/redux",
   "devDependencies": {
     "babel": "^5.5.8",
-    "babel-core": "^5.5.8",
+    "babel-core": "5.6.15",
     "babel-eslint": "^3.1.15",
     "babel-loader": "^5.1.4",
     "eslint": "^0.23",