#203 mute error output on yarn packager check (Windows)

Summary:
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- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.

## Motivation (required)

Fixes #203 wherein the check for `yarn` outputs extraneous error messaging. This mutes the error output on Windows.

## Test Plan (required)

#BrokenWindows here, cli doesn't have tests and I don't want to presume to design a testing setup for it, considering the JS side wouldn't be very knowledgeable about this change anyhow.

I hope the following demonstrates the fix:

![image](https://cloud.githubusercontent.com/assets/4138357/26280295/88364a20-3d92-11e7-873e-2301dd5a3e08.png)

## Next Steps

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

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For more info, see the ["Pull Requests"][4] section of our "Contributing" guidelines.

*** Of note, lint is failing on non-cli stuff. I can pick those up separately.

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: http://circleci.com/gh/facebook/react-native
[4]: https://github.com/facebook/react-vr/blob/master/CONTRIBUTING.md#pull-requests
Closes #204

Reviewed By: andrewimm

Differential Revision: D5119875

Pulled By: andrewimm

fbshipit-source-id: 1654f96

react-vr-cli/src/getPackager.js

@@ -17,7 +17,7 @@ function getPackager() {
   }
   try {
     child_process.execSync(
-      /^win/.test(process.platform) ? 'yarn --version' : 'yarn --version 2>/dev/null'
+      /^win/.test(process.platform) ? 'yarn --version 2> NUL' : 'yarn --version 2>/dev/null'
     );
     packager = 'yarn';
   } catch (e) {