Correct invalid byte sequence with check-log.rb

The log should be parsed with ISO-8859-1 encoding.

    root@ds1217:/var/log/upstart# grep --color='auto' -P -n "[\x80-\xFF]"  /var/log/upstart/neutron-server.log
    1878:10.100.2.83 - - [21/Nov/2013 22:30:33] "��R����,��EM�4r0��y�?��{��eL�?}x��" 400 -
    1881:10.100.2.83 - - [21/Nov/2013 22:30:41] "��R�������D���v�y��w������P#n���x��" 400 -

    root@ds1217:/var/log/upstart# /etc/sensu/plugins/check-log.rb -f /var/log/upstart/neutron-server.log -q ERROR
    CheckLog CRITICAL: Check failed to run: invalid byte sequence in UTF-8, ["/etc/sensu/plugins/check-log.rb:117:in `match'", "/etc/sensu/plugins/check-log.rb:117:in `match'", "/etc/sensu/plugins/check-log.rb:117:in `block in search_log'", "/etc/sensu/plugins/check-log.rb:115:in `each_line'", "/etc/sensu/plugins/check-log.rb:115:in `search_log'", "/etc/sensu/plugins/check-log.rb:80:in `run'", "/var/lib/gems/1.9.1/gems/sensu-plugin-0.2.2/lib/sensu-plugin/cli.rb:56:in `block in <class:CLI>'"]