Use `ltx` from node-xmpp-core

See https://github.com/node-xmpp/node-xmpp-client/issues/42
...and https://github.com/node-xmpp/node-xmpp-core/pull/39
Fix all references to ltx
Merge pull request #43 from node-xmpp/ltx

Use `ltx` from node-xmpp-core