Update for 0.9.13

Technically this updates to Graphite 0.9.13-pre1, but that's close enough to warrant a fresh release. There are a number of fixes here, too.

• fixes for admin static files
• change deps to cairocffi
• add DigitalOcean install button to README (@deniszh)
• redirect http to https
• expose Carbon udp listener (@steveworkman)
• runtime directory fix (@mathyourlife)
• relax Ubuntu release check (@Dispader)
• set Graphite default version in Vagrant
• ensure carbon-cache is terminated before removing carbon user during uninstall
• repair database fixture
• remove explicit support for 0.9.x branch
• cron for rebuilding search index
• improved WSGI setup for Apache
• set carbon-cache initscript defaults so Carbon starts after reboot