Fix warnings in configurable

This fixes the following warnings:

```
octokit-2.6.2/lib/octokit/configurable.rb:92: warning: method redefined; discarding old api_endpoint
octokit-2.6.2/lib/octokit/configurable.rb:99: warning: method redefined; discarding old web_endpoint
octokit-2.6.2/lib/octokit/configurable.rb:103: warning: method redefined; discarding old login
```