updated dependencies (core and test)

Using our own php-ixr fork until the following PRs are merged upstream:

https://github.com/kissifrot/php-ixr/pull/13
https://github.com/kissifrot/php-ixr/pull/14

updated kissifrot/php-ixr closes #4429

updated composer dependencies

updates may also be caused by the php platform version increase in the last
commit

move IXR XML RPC to composer dependency #1970

This is a monky patched version of kissyfrot/php-ixr until my PR has
been accepted upstream.

Plugins using the XML RPC client may need adjustments.