Fix issues with prepare-dev.py and rb-site.
There were a couple issues in running prepare-dev.py with django 1.6. The first

one was that rb-site was still using execute_manager, which has been deprecated

roughly forever. That has finally been removed, so we need to switch to

execute_from_command_line. Second, we weren't getting a useful error message,

because prepare-dev was invoking commands in rb-site without the 'ui' variable

being set to anything.

Ran prepare-dev.py successfully.