Add forgotten return to get_main_iface()
Add forgotten return to get_main_iface()

This should fix the following backtrace I get when I switch to develop:

```
2015-02-13 11:34:42,521 qtile init_log:93  Starting Qtile
2015-02-13 11:34:42,979 qtile safe_import:49  Can't Import Widget: '.launchbar.LaunchBar', No module named xdg.IconTheme
2015-02-13 11:34:42,981 qtile safe_import:49  Can't Import Widget: '.mpriswidget.Mpris', No module named dbus
2015-02-13 11:34:42,982 qtile safe_import:49  Can't Import Widget: '.mpris2widget.Mpris2', No module named dbus
2015-02-13 11:34:42,983 qtile safe_import:49  Can't Import Widget: '.mpdwidget.Mpd', No module named mpd
2015-02-13 11:34:42,994 qtile safe_import:49  Can't Import Widget: '.wlan.Wlan', No module named pythonwifi.iwlibs
2015-02-13 11:34:43,007 qtile safe_import:49  Can't Import Widget: '.google_calendar.GoogleCalendar', ado/nobackup/virtualenvs/qtile/lib/python2.7/site-packages (from qtile==0.9.0)
No module named dateutil.parser
2015-02-13 11:34:43,008 qtile safe_import:49  Can't Import Widget: '.imapwidget.ImapWidget', No module named keyring
2015-02-13 11:34:43,013 qtile make_qtile:109  Error while reading config file
Traceback (most recent call last):
  File "./bin/qtile", line 107, in make_qtile
    c = confreader.File(options.configfile, is_restart=options.no_spawn)
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/confreader.py", line 63, in __init__
    raise ConfigError(str(v) + "\n\n" + tb)
ConfigError: 'NoneType' object has no attribute '__getitem__'

Traceback (most recent call last):
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/confreader.py", line 53, in __init__
    config = __import__(os.path.basename(fname)[:-3])
  File "/afs/dfn-cert.de/user/delgado/.config/qtile/config.py", line 357, in <module>
    ] + _barend(),
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 318, in __init__
    self.interface = self.get_main_iface()
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 355, in get_main_iface
    routes[0]
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 354, in <genexpr>
    (r for r in routes if not int(r['dest'], 16)),
TypeError: 'NoneType' object has no attribute '__getitem__'

Traceback (most recent call last):
  File "./bin/qtile", line 130, in <module>
    q = make_qtile()
  File "./bin/qtile", line 107, in make_qtile
    c = confreader.File(options.configfile, is_restart=options.no_spawn)
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/confreader.py", line 63, in __init__
    raise ConfigError(str(v) + "\n\n" + tb)
ConfigError: 'NoneType' object has no attribute '__getitem__'

Traceback (most recent call last):
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/confreader.py", line 53, in __init__
    config = __import__(os.path.basename(fname)[:-3])
  File "/afs/dfn-cert.de/user/delgado/.config/qtile/config.py", line 357, in <module>
    ] + _barend(),
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 318, in __init__
    self.interface = self.get_main_iface()
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 355, in get_main_iface
    routes[0]
  File "/afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/widget/graph.py", line 354, in <genexpr>
    (r for r in routes if not int(r['dest'], 16)),
TypeError: 'NoneType' object has no attribute '__getitem__'


> /afs/dfn-cert.de/user/delgado/nobackup/anon-vc/qtile/bin/libqtile/confreader.py(63)__init__()
-> raise ConfigError(str(v) + "\n\n" + tb)
(Pdb)
```

I'll test this locally and then file a pull request.
kopchik/qtile,himaaaatti/qtile,rxcomm/qtile,StephenBarnes/qtile,soulchainer/qtile,tych0/qtile,w1ndy/qtile,soulchainer/qtile,apinsard/qtile,xplv/qtile,zordsdavini/qtile,dequis/qtile,kseistrup/qtile,jdowner/qtile,zordsdavini/qtile,rxcomm/qtile,farebord/qtile,flacjacket/qtile,kynikos/qtile,dequis/qtile,cortesi/qtile,frostidaho/qtile,kseistrup/qtile,nxnfufunezn/qtile,nxnfufunezn/qtile,kynikos/qtile,jdowner/qtile,qtile/qtile,xplv/qtile,ramnes/qtile,qtile/qtile,apinsard/qtile,kopchik/qtile,frostidaho/qtile,w1ndy/qtile,aniruddhkanojia/qtile,kiniou/qtile,de-vri-es/qtile,tych0/qtile,EndPointCorp/qtile,ramnes/qtile,farebord/qtile,kiniou/qtile,himaaaatti/qtile,cortesi/qtile,andrewyoung1991/qtile,de-vri-es/qtile,EndPointCorp/qtile,flacjacket/qtile,andrewyoung1991/qtile,StephenBarnes/qtile,aniruddhkanojia/qtile