Make fix_locales work on Debian

I found the problems we were having with locales not getting fixed by BOA: on Debian, locale-gen does not take any arguments (this is Ubuntu-specific) and you have to edit /etc/locale.gen instead. See issues #351 and #396.

I've also used `update-locale LANG=en_US.UTF-8` instead of echoing into /etc/default/locale. This works at least on Debian, not sure about Ubuntu so I haven't changed that. I've omitted localedef, since according to the man page this is already called by locale-gen (and this seems to work).