IE doesn't support overrideMimeType

Fixes the GEXF parsing plugin for IE. Without this the script will stop at a hard error on the following line:

```javascript
gexfhttp.overrideMimeType('text/xml');
```

The fix is to just put it in a block:

```javascript
if (gexfhttp.overrideMimeType) {
  gexfhttp.overrideMimeType('text/xml');
}
```