add an User-Agent in order to get a response from github

Running `projects github` to populate the database took a lot of time, then I discovered that Github will reply with the following response-body if you are not specifying any 'User-Agent' header:

```
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.
```