Add missing require

I received an error when calling `Cloudinary::Api.resources`:

```
NameError: uninitialized constant Cloudinary::Api::RestClient
  from /Users/manuel/code/cloudinary_gem/lib/cloudinary/api.rb:112:in `call_api'
  from /Users/manuel/code/cloudinary_gem/lib/cloudinary/api.rb:33:in `resources'
```

Adding a `require 'rest_client'` to the top of `lib/cloudinary/api.rb` fixed this for me.