It seems you might have multiple versions of libxml2 installed.
First you could try to upgrade your libxml2.
I'm fairly sure all the humble-bundle games come with a launch script to load the bundled libraries instead of looking them from your system, so you could try launching the game with that.
If this doesen't help either see if you indeed have multiple installations of libxml on your machine with "locate libxml2.so.2" (without quotes) from terminal. It should be located (atleast) in /usr/local/lib/libxml2.so.2, but if you have multiple ones you can delete the old one and create symbolic link to the new one with the command ln -s
I'm not 100% on this but i'll test this more once i get home to my linux machine.