Update meta.js (new Buffer when init)

Summary:
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Warning about new Buffer when init
Prefer **small pull requests**. These 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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Make sure tests pass on both Travis and Circle CI.

**Code formatting**

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Closes https://github.com/facebook/react-native/pull/11153

Differential Revision: D4234591

fbshipit-source-id: a9373f3151de8a50c18342f45fb9684a5c7416a6

local-cli/bundle/output/meta.js

@@ -23,7 +23,7 @@ module.exports = function(code, encoding) {
   const hash = crypto.createHash('sha1');
   hash.update(code, encoding);
   const digest = hash.digest('binary');
-  const signature = Buffer(digest.length + 1);
+  const signature = Buffer.alloc ? Buffer.alloc(digest.length + 1) : new Buffer(digest.length + 1); // remove the new Buffer call when RN drops support for Node 4
   signature.write(digest, 'binary');
   signature.writeUInt8(
     constantFor(tryAsciiPromotion(code, encoding)),