Fix bug in using array_flip with PHP5.3

When running on the following environment:
PHP Version 5.3.10-1ubuntu3.8
Linux 3.2.0-54-generic #82-Ubuntu SMP Tue Sep 10 20:08:42 UTC 2013 x86_64 
Apache/2.2.22 (Ubuntu) 

I was getting the following error:
[Tue Oct 01 21:30:05 2013] [error] [client 108.162.245.233] PHP Parse error:  syntax error, unexpected '(', expecting ',' or ';
' in /home/main/hotcrp/lib/cleanhtml.php on line 8, referer: http://raymondcheng.net/hotcrp/settings.php?group=rfo

The following patch just replaces using isset on a dictionary to just searching over an array with in_array. Works good for me now. Use it as you wish.

Thanks for maintaining this awesome project!