composer.json - Update civicrm-cxn-rpc for PHP 7

composer.json - Update civicrm-cxn-rpc for PHP 7

To work with PHP 7, civicrm-cxn-rpc requires a newer version of phpseclib.

Note: civicrm-cxn-rpc includes a test suite, which we've checked in PHP 5.3,
5.4, and 7.0. `civicrm-core` doesn't seem to have much test coverage here,
so I did a manual test run locally (MAMP / PHP 5.5), and these steps worked:

 * Setup https://github.com/civicrm/civicrm-cxn-rpc/blob/master/doc/proxy.md
 * Connect to "Site Profile" service
 * View "Settings" and view the profile
 * Disconnect