File comparison using ruby

I need to compare 2 different files and show the difference between those two files.

I looked it up online and got this answer from http://ift.tt/Uo4Bsm but i am getting this error message

NameError: uninitialized constant Diffy

from (irb):11

from /usr/bin/irb:12:in `<main>'

Has someone already encountered this problem before? THX