Added the --allow-root to bower install.

It looks like in newer bower versions (1.2.5 in my dev server) bower install will fails when running it as root.
There is the new --allow-root option that will prevent this error.
I have successfully run paver update_static with this pavement file both with a new (1.2.5) and old (0.9.2) bower version, so it should be safe to accept this modification.