Update to node-expat 2.0.0

The 2.0.0 version of node-expat bundles the libexpat dependency (https://github.com/astro/node-expat/pull/48) which makes it easier for both Windows / Unix users to get node-expat installed.

Here's a link to the full diff between node-expat v1.6.1 and v2.0.0 - https://github.com/astro/node-expat/compare/v1.6.1...v2.0.0