Bump version of node-expat

The latest version of `node-expat` adds support for node 0.11.13 and up. However, this drops support for previous versions of 0.11 (see https://github.com/node-xmpp/node-expat/issues/90#issuecomment-44088967). Still works with the latest v 0.10.x release.

Closes #11
does this only work with a certain 0.10 and above?  If so we should be explicit about the version