Stephen Gallagher's recent change to install sites into a default directory had

the unintentional side effect of breaking prepare-dev.py. The issue was that

using the Site object with a relative path now tries to use that path relative

to the default directory rather than the current working directory.
This change makes the relative 'reviewboard' path absolute before constructing

the Site object.

Ran prepare-dev.py. Before this change it would error out with permissions.

After, it runs successfully.