Check openssl extension for proxy

src/Composer/Downloader/FileDownloader.php

@@ -64,6 +64,10 @@ abstract class FileDownloader implements DownloaderInterface
             // http(s):// is not supported in proxy
             $proxy = str_replace(array('http://', 'https://'), array('tcp://', 'ssl://'), $_SERVER['HTTP_PROXY']);
 
+            if (0 === strpos($proxy, 'ssl:') && !extension_loaded('openssl')) {
+                throw new \RuntimeException('You must enable the openssl extension to use a proxy over https');
+            }
+
             $ctx = stream_context_create(array(
                 'http' => array(
                     'proxy'           => $proxy,