Update link to Bower Server

The link to Bower Server was stale. Inspecting the various [repositories][repos] available under bower gave a likely replacement candidate, i.e. [registry][].

Inspecting the [application.rb][registry-application] reveals the former name in the comments, confirming my assumptions about the correct link destination.

[repos]: https://github.com/bower
[registry]: https://github.com/bower/registry
[registry-application]: https://github.com/bower/registry/blob/master/application.rb#L2