Backward incompatible change in exception namespace change

It looks like the top level namespace for exceptions is changed from `RightApi::Exceptions` to just `RightApi`. The commit 84f477907eef0a583ee5bec0ee5336309d933c75 added partial support for backward compatibility but this new module is not included by default when including `right_api_client` and the consumer code will have to include `right_api_client/exceptions` explicitly. 

To make this change completely backward compatible, the `lib/right_api_client/client.rb` should require `right_api_client/exceptions`.