Prevent warning from being printed in gpg2.1

The ignored --secret-keyring option causes a warning to be printed
to the user.

See: https://dev.gnupg.org/T2749

lib/Horde/Crypt/Pgp/Backend/Binary.php

@@ -792,7 +792,7 @@ protected function _createKeyring($type = 'public')
             if (empty($this->_privateKeyring)) {
                 $this->_privateKeyring = $this->_createTempFile('horde-pgp');
             }
-            return '--secret-keyring ' . $this->_privateKeyring;
+            return ($this->_gnupg21 ? '' : '--secret-keyring ') . $this->_privateKeyring;
         }
     }