Bump node-expat version from ~1.4.0 to ~2.0.0

Here's a patch to update node-expat to 2.0.0. It's like the one you already received from marcelduran@7af5c647, but sticks with `~` instead of `>=`.

[Link to discussion](https://github.com/raszi/node-jsonml/pull/2#issuecomment-12679944)