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.

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_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._
I'm seeing

> PHP Parse error:  syntax error, unexpected 'public' (T_PUBLIC) in /home/travis/build/codeclimate/php-test-reporter/src/CodeClimate/Bundle/TestReporterBundle/Command/TestReporterCommand.php on line 86

on Travis.

I *think* you're missing the closing brace for the `switch`.