Explicitly require json

Was receiving this error when running the example app test suite:

```terminal
Failures:

  1) API Examples add_contact_address (v 2.0) has valid data for example 1
     Failure/Error: define_test(description) { example.validate! }
     NoMethodError:
       undefined method `parse' for JSON:Module
     # /Users/harlow/opensource/interpol/lib/interpol/endpoint.rb:39:in `open'
```

* Added `json` require
* Sorted require statements alphabetically