Add semi-colon character to configparser whitelist

semi-colons are used in "ticket" key-values pairs to
apply the the ticket-expiry timeout in seconds.

Fixes issues for configuratoin like this:

ticket="ticketA;1000"

$ /usr/sbin/booth arbitrator -D
1325254499 invalid character (';', lineno 6) in config file

Signed-off-by: Daniel Gollub <gollub@b1-systems.de>