Use Walmart's App Store icon.

Summary:
This fixes an issue where Walmart's icon is not showing up in production. I added the icon to the RN website, but this goes against convention which is to use an externally hosted image for the showcase.

This PR uses the App Store icon for the app.

Before:

<img width="1069" alt="unknown" src="https://cloud.githubusercontent.com/assets/165856/21729495/3bb95990-d400-11e6-98b9-9ffd86a1b3b6.png">

After:
![screen shot 2017-01-06 at 11 01 31 am](https://cloud.githubusercontent.com/assets/165856/21729508/44d049bc-d400-11e6-81ff-87c9550a8ea6.png)
Closes https://github.com/facebook/react-native/pull/11754

Differential Revision: D4387761

Pulled By: ericvicenti

fbshipit-source-id: 1be4903c1f05be2cfd8403f8746c1917669d196f

website/src/react-native/index.js

@@ -76,7 +76,7 @@ var apps = [
   },
   {
     name: 'Walmart',
-    icon: 'http://facebook.github.io/react-native/img/showcase/wmt_spark.png',
+    icon: 'http://is2.mzstatic.com/image/thumb/Purple111/v4/64/9f/20/649f2026-e968-0417-660c-e5ee6d7977ff/source/350x350bb.jpg',
     infoLink: 'https://itunes.apple.com/us/app/walmart-app-shopping-savings/id338137227?mt=8',
   },
 ];

website/src/react-native/showcase.js

@@ -430,7 +430,7 @@ var pinned = [
   },
   {
     name: 'Walmart',
-    icon: 'http://facebook.github.io/react-native/img/showcase/wmt_spark.png',
+    icon: 'http://is2.mzstatic.com/image/thumb/Purple111/v4/64/9f/20/649f2026-e968-0417-660c-e5ee6d7977ff/source/350x350bb.jpg',
     linkAppStore: 'https://itunes.apple.com/us/app/walmart-app-shopping-savings/id338137227?mt=8',
     linkPlayStore: 'https://play.google.com/store/apps/details?id=com.walmart.android&hl=en',
     infoLink: ' https://medium.com/walmartlabs/react-native-at-walmartlabs-cdd140589560#.ueonqqloc',