Fix rake task for update select2 files

I added a fake user_agent - otherwise you will get the following error if u run the rake task `update-select2`:
```
JSON::ParserError: 757: unexpected token at 'Request forbidden by administrative rules. Please make sure your request has a User-Agent header (http://developer.github.com/v3/#user-agent-required). Check https://developer.github.com for other possible causes.
```