Check for proper "github" module

Caused by Melissa's having PyGithub3 installed.

```
$ python scc.py rebase 334 develop
Traceback (most recent call last):
  File "scc.py", line 1481, in <module>
    main()
  File "scc.py", line 1476, in main
    ns.func(ns)
  File "scc.py", line 1251, in __call__
    self.login(args)
  File "scc.py", line 1019, in login
    self.gh = get_github(token, dont_ask=args.no_ask)
  File "scc.py", line 150, in get_github
    return GHManager(login_or_token, password, **kwargs)
  File "scc.py", line 170, in __init__
    except github.GithubException, ge:
AttributeError: 'module' object has no attribute 'GithubException'
```