Turn off sendfile in development

Virtualbox has a bug that could cause static files to not update and
seem cached. References:

* http://wiki.nginx.org/Pitfalls (Virtualbox section)
* http://jeremyfelt.com/code/2013/01/08/clear-nginx-cache-in-vagrant/
* https://github.com/mitchellh/vagrant/issues/351#issuecomment-1339640