composer.json - Update civicrm-cxn-rpc and phpseclib

composer.json - Update civicrm-cxn-rpc and phpseclib

Before
------

Require `civicrm-cxn-rpc` v0.19 (with `phpseclib` v1.x)

After
-----

Require either of:

* `civicrm-cxn-rpc` v0.20 (with `phpseclib` v2.x)
* `civicrm-cxn-rpc` v0.19 (with `phpseclib` v1.x)

Technical Details
-----------------

* The public interfaces from civicrm-cxn-rpc are the same in 0.19+0.20.
  They only differ in which verison of `phpseclib` is used.
* As pointed out in https://github.com/civicrm/civicrm-cxn-rpc/issues/9, we're not
  the only folks using phpseclib, so some flexibility on that seems good.
* The primary change in phpseclib 2.x is the use of PHP namespaces
  (e.g. `Crypt_AES` => `\phpseclib\Crypt\AES`).
* There are newer versions of both v0.19 and v0.20 which bundle an updated certificate.