added trailing slash to rsyslog::server's $server_dir default

When I applied `rsyslog::server`, with defaults, logs were ending up as `/srv/log<srcname>/file…` rather than `/srv/log/<srcname>/file…`.  I presume this is incorrect?

Ubuntu 12.04.2, rsyslog package 5.8.6-1ubuntu8.
Merge pull request #35 from markhellewell/patch-1

added trailing slash to rsyslog::server's $server_dir default