Removed Gitter link. Seems to be inactive.

README.md

@@ -18,7 +18,6 @@ The aim of the project is to create an easy to use, lightweight, 3D library with
 [Migrating](https://github.com/mrdoob/three.js/wiki/Migration-Guide) —
 [Questions](http://stackoverflow.com/questions/tagged/three.js) —
 [Forum](https://discourse.threejs.org/) —
-[Gitter](https://gitter.im/mrdoob/three.js) —
 [Slack](https://join.slack.com/t/threejs/shared_invite/enQtMzYxMzczODM2OTgxLTQ1YmY4YTQxOTFjNDAzYmQ4NjU2YzRhNzliY2RiNDEyYjU2MjhhODgyYWQ5Y2MyZTU3MWNkOGVmOGRhOTQzYTk)
 
 ### Usage ###