Multiple reviewed PR's for Github script throws script error

if there's a single PR per person then the script runs ok as python seems to compensate for lists with 1 item, however if there are multiple then we get a script execution error

```
Traceback (most recent call last):
  File "./github-stats.py", line 266, in <module>
    print "Reviewed Pull Requests/GH{0}/{1}/{2}".format(issue_value['id'], key, 1)
KeyError: 'issue_value'
```