Update scratchr2_settings.py

(Hopefully) adding Feature #1211 (Bugtracker): http://scratchexp.media.mit.edu/issues/1211
-> should add modshare.futuresight.org and cubeupload.com to the server white list for images in the forums

(well, I really hope this request-think works, as far as I know my first pull request wasn't in fact a real pull request or I don't know, I guess I don't understand Github yet^^. Well anyways, if this doesn't work, the code I want to suggest is here: https://github.com/LiFaytheGoblin/s2forums/blob/master/djangobb_forum/scratchr2_settings.py)