Fix for displayName transform

Summary: The `react.displayName` transform was added in 93b9329b75.

That diff missed to update the `.babelrc` where the comment says it should stay
in sync (I'm not sure where it's used though). I added a comment in the other
direction so this can be prevented in the future.

I also updated the `cacheVersion` so we actually transform the code again to add
the missing displayName properties to unchanged components.
Closes https://github.com/facebook/react-native/pull/2905

Reviewed By: @vjeux

Differential Revision: D2473447

Pulled By: @kassens

packager/packager.js

@@ -291,7 +291,7 @@ function getAppMiddleware(options) {
     nonPersistent: options.nonPersistent,
     projectRoots: options.projectRoots,
     blacklistRE: blacklist(),
-    cacheVersion: '2',
+    cacheVersion: '3',
     transformModulePath: transformerPath,
     assetRoots: options.assetRoots,
     assetExts: ['png', 'jpeg', 'jpg'],