Custom phantomjs download url.

User can set `PHANTOMJS_CDNURL` env to download phantomjs from
other cdn.

Because `http://cdn.bitbucket.org` was block by GFW in China, so we want to download phantomjs bin from our mirror site.

e.g.:

```bash
$ PHANTOMJS_CDNURL=http://cnpmjs.org/downloads node install

Downloading http://cnpmjs.org/downloads/phantomjs-1.9.7-macosx.zip
Saving to /var/folders/5h/x4h0t8rs6w300g_wnzbx47km0000gn/T/phantomjs/phantomjs-1.9.7-macosx.zip
Receiving...
```