use PATH_INFO instead of REQUEST_PATH

I'm actually not completely positive about this, but my research seems to indicate that PATH_INFO is the standard key for the value that is being used here. When testing unicorn, it seems that PATH_INFO and REQUEST_PATH are equal, but using Trinidad (and possibly other app servers) I don't think there is a guarantee that REQUEST_PATH will be set properly.

http://rack.rubyforge.org/doc/SPEC.html