Update composer.json

They fixed in 3.8.1 this problem:

Warning: curl_multi_info_read(): 560 is not a valid cURL Multi Handle resource in Guzzle\Http\Curl\CurlMulti->processMessages() (line 241 of /vendor/guzzle/guzzle/src/Guzzle/Http/Curl/CurlMulti.php).
Merge pull request #8 from startupz/patch-1

Update composer.json