Update gitter badge

Fixes #314

Author: kevmoo

README.md

@@ -7,8 +7,8 @@
   <a href="https://pub.dartlang.org/packages/source_gen">
     <img src="https://img.shields.io/pub/v/source_gen.svg" alt="Pub Package Version" />
   </a>
-  <a href="https://gitter.im/dart-lang/source_gen">
-    <img src="https://badges.gitter.im/dart-lang/source_gen.svg" alt="Join the chat on Gitter" />
+  <a href="https://gitter.im/dart-lang/build">
+    <img src="https://badges.gitter.im/dart-lang/build.svg" alt="Join the chat on Gitter" />
   </a>
 </p>