Fixing undefined variable

I was getting this exception every time I connected to HipChat:

> ERROR    Error processing event handler: <bound method WillBot.join_rooms of <will.main.WillBot object at 0x1530450>>
> Traceback (most recent call last):
>   File "/home/austin/.virtualenvs/hipbot/local/lib/python2.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 1693, in > _event_runner
>     func(*args)
>   File "/home/austin/src/hipbot/will-amckinley/will/listener.py", line 55, in join_rooms
>     self.update_will_roster_and_rooms()
>   File "/home/austin/src/hipbot/will-amckinley/will/listener.py", line 84, in update_will_roster_and_rooms
>     user_data = self.full_hipchat_user_list[hipchat_id]
>   File "/home/austin/src/hipbot/will-amckinley/will/mixins/hipchat.py", line 99, in full_hipchat_user_list
>     r = requests.get(url, params=params)
> NameError: global name 'params' is not defined