Add 'dbconfig' to requirements.txt

Created by: CristianCantoro

Launching app.py I get the following error:

$ python app.py 
Traceback (most recent call last):
  File "app.py", line 42, in <module>
    from userstats import UserStats
  File "/media/cristian/data/extra/oabot/oabot/userstats.py", line 7, in <module>
    from dbconfig import get_engine
ImportError: No module named dbconfig

the module dbconfig needs to be installed, so I add it to requirements.txt.