Upgrade PostgreSQL 9.1 to 9.3 and server no longer starts

Technically, I didn't upgrade from 9.1 to 9.3, I built a new server with Ubuntu 14.04 and used my old 9.1 config files. PostgreSQL wouldn't start:

* Starting PostgreSQL 9.3 database server
* The PostgreSQL server failed to start. Please check the log output.

but the logs were empty.

Eventually, after comparing the stock 9.3 postgresql.conf file and my 9.1 file I figured out that, at some point, the config variable unix_socket_directory had been changed to unix_socket_directories (not that easy to pick up in a diff with many changes and comments after each line).

Posted because I couldn't find anything on the net about this (until after I already knew what the issue was).