Running bin/make_css after install script

As reported at start of https://secure.mysociety.org/admin/lists/pipermail/fixmystreet/2013-May/000101.html

The install script sets GEM_HOME, GEM_PATH, and adds GEM_HOME/bin to PATH, and then runs make_css; but if you want to run make_css later, you don't have those settings any more. Not sure what's the best approach here - add those envvars to the user, or add to cron-wrapper in some way, for consistency I guess?

(Also, having hard-coded list of sites in make_css isn't great, should do that better.)