add missing dependency: pil

I was getting the confusing error "Cairo is not available" even though it is installed. Then I looked at the stack the trace and the problem was here:
https://github.com/tauberer/represent-maps/blob/master/maps/views.py#L15

I was missing PIL. I figured the easiest way to resolve the problem was to "pip install" it. I don't know if PIL comes with python by default or something in Ubuntu, but it apparently doesn't come with the python2 package for Arch Linux and who knows what other distros.