Swap Discord badge for shields.io version

This brings its style inline with all the other badges.

README.md

@@ -12,7 +12,7 @@
   <a href="https://travis-ci.org/yarnpkg/yarn"><img alt="Travis Status" src="https://travis-ci.org/yarnpkg/yarn.svg"></a>
   <a href="https://circleci.com/gh/yarnpkg/yarn"><img alt="Circle Status" src="https://circleci.com/gh/yarnpkg/yarn.svg?style=shield&circle-token=5f0a78473b0f440afb218bf2b82323cc6b3cb43f"></a>
   <a href="https://ci.appveyor.com/project/kittens/yarn/branch/master"><img alt="Appveyor Status" src="https://ci.appveyor.com/api/projects/status/0xdv8chwe2kmk463?svg=true"></a>
-  <a href="https://discord.gg/yarnpkg"><img alt="Discord Chat" src="https://discordapp.com/api/guilds/226791405589233664/widget.png"></a>
+  <a href="https://discord.gg/yarnpkg"><img alt="Discord Chat" src="https://img.shields.io/discord/226791405589233664.svg"></a>
 </p>
 
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