Update pystache usage to pystache>=0.5.0 (current release)

The usage of the pystache dependency has been updated to >=0.5.0.  Tested with 0.5.3 and 0.5.4 (the current release).

Frankly, I'm not sure why pystache is employed at all; Askbot already has two other templating engines at its disposal (Jinja2 and Django's), and it only uses pystache for the trivial askbot-setup script.  It would make sense to remove pystache altogether.

For now, though, the pystache version was conflicting with the latest version in use on my project, so I just updated Askbot to use the latest version.