Upgrade xtend

This would allow deduping `xtend` in geojson-mapnikify:

```
$ npm ls xtend
geojson-mapnikify@0.4.3 /Users/dane/projects/geojson-mapnikify
├─┬ makizushi@1.2.0
│ └── xtend@3.0.0 
├─┬ mapnik@3.3.0
│ └─┬ node-pre-gyp@0.6.7
│   └─┬ request@2.55.0
│     └─┬ har-validator@1.7.0
│       └─┬ is-my-json-valid@2.10.1
│         └── xtend@4.0.0 
└── xtend@4.0.0 
```
