Requiring builder explicitly.

This fixes an issue in the Rails 3 build of Voice Core where use of `call_center` throws an error:
```
 NameError:   uninitialized constant CallCenter::Builder
```
...from here...
https://github.com/zendesk/call_center/blob/master/lib/call_center.rb#L28

I've verified this works locally and ideally I will go ahead and do a `call_center` version bump to `1.0.2` once this PR is approved.

// cc @hsume2 @GavinJoyce