Use cURL extension instead of Guzzle

The only HTTP client operation in this library is to transfer the resulting JSON data in a trivial HTTP POST request with minimal error handling (can't do anything besides outputting the error to CLI).

This PR…

1. Replaces the dependency on Guzzle with a dependency on the PHP cURL extension.
1. Changes `ApiClient` to return a very simplistic response object.
1. Adjusts `TestReporterCommand` accordingly.

---
_Note: An alternative implementation using PHP HTTP/SSL stream contexts would be possible, but requires additional code to cleanly handle PHP warnings possibly thrown by `fopen()`, unless error handling is removed altogether._