Tweak version of CSS in dev

I changed the gittip.css simplate to 404 if the version requested doesn't
match the current version of the app, so that we don't accidentally
serve CSS that's out of sync with our markup. Without this we'd actually
have a potential attack vector, in that someone could prefill our CDN
cache with the current version of the CSS by requesting future versions
from the CDN that would be served with the current version by the origin
server.

For development we don't want to require sass out of the box, and so we
want to be able to point to a CDN-hosted CSS file by default. Since the
version in development won't match the version in production, I added an
out: if you specify '-' as the version you won't get a 404, you'll get
the file. This changes default local.env to take advantage of that.