Update src/Composer/Downloader/ArchiveDownloader.php

Fix for single-file archives, for example http://sourceforge.net/projects/rssphp/files/rssphp/v1/rss_php.v1.zip/download

src/Composer/Downloader/ArchiveDownloader.php

@@ -58,7 +58,13 @@ abstract class ArchiveDownloader extends FileDownloader
                         rename($file, $path . '/' . basename($file));
                     }
                 }
-                rmdir($contentDir);
+                
+                if(is_file($contentDir)){
+                    unlink($contentDir);
+                }
+                else{
+                    rmdir($contentDir);
+                }
             }
         } catch (\Exception $e) {
             // clean up