rum_runner says it uses a default --prefix, but it doesn't

[amirkarger]

Version 2.0.5_06

The man page for rum_runner says:

By default it will operate on the indexes installed at the same
location as this script. For example, if the script is located at
C</rum/bin/rum_indexes>, it will assume that config files are in
C</rum/conf> and indexes are in C<~/rum/indexes>. You can tell it to
use another location with the B<--prefix> option.

However, the code dies with an error message if you don't input --prefix.