Changing build path in package.json will not properly create directories to link plugin
If i specify a nested build path inside my `package.json` file, it will not create the proper directories inside the Sketch app directory to symlink the plugin

i.e. 
```
{
  "skpm": {
    "name": "trade-flow-sketch-plugin",
    "manifest": "sketch/manifest.json",
    "main": "build/plugin.sketchplugin"
  }
}
```

```
[1/1] 🔗  Symlinking the plugin trade-flow-sketch-plugin...
error Error while symlinking the plugin trade-flow-sketch-plugin
{ Error: ENOENT: no such file or directory, symlink '/Users/calvinchan/airswap/trade-flow/build/plugin.sketchplugin' -> '/Users/calvinchan/Library/Application Support/com.bohemiancoding.sketch3/Plugins/trade-flow-sketch-plugin/build/plugin.sketchplugin'
```

If I remove the `build/` directory from package.json, it symlinks fine