[ReactNative] Pin babel version

Summary:
Currently minor version babel updates add and remove transforms, but internal
version is checked in and pinned to 5.6.4. Until we figure out how to update
internal deps systematically, we need to make sure OSS edition of RN matches
internal, otherwise we get test failures due to package version mismatches.

package.json

@@ -48,7 +48,7 @@
   "dependencies": {
     "absolute-path": "0.0.0",
     "babel": "5.4.3",
-    "babel-core": "^5.6.4",
+    "babel-core": "5.6.4",
     "chalk": "^1.0.0",
     "connect": "2.8.3",
     "debug": "~2.1.0",