Upgrade node-expat to 2.1.4

This will allow compiling xml2json on newer versions of Node, such as v0.11.x and above.

I ran the existing tests and they all pass.

```bash
[xml2json: array-notation.xml->array-notation.json] passed!
[xml2json: coerce.xml->coerce.json] passed!
[json2xml: domain-reversible.json->domain.xml] passed!
[xml2json: domain.xml->domain.json] passed!
[xml2json: large.xml->large.json] passed!
[xml2json: reorder.xml->reorder.json] passed!
[xml2json: spacetext.xml->spacetext.json] passed!
```