Fix "docpad is not defined" error on Docpad v6.66.0

Due to recent changes to Docpad (v6.66.0), thumbnail plugin stopped working giving the following error:
 ```
warning: Something went wrong while rendering: post.html.coffee
ReferenceError: docpad is not defined
  at Object.templateData.getThumbnail (/home/angel/Workspace/UnivUnix-RPi/node_modules/docpad-plugin-thumbnails/out/thumbnails.plugin.js:107:9)
```
To fix this error, I added the missing reference.

Regards.