Support Ruby 1.8

This should fix #150
I don't think this is a good idea. Here's why:

1. You're adding `require_relative` to the `$LOAD_PATH` by putting it in lib.  This means anyone using the gem can `require 'require_relative'` and load this code.  That would be surprising.  Assuming we need this hack, it should be elsewhere.  I would recommend copying steve's code directly into `vendor/require_relative` since it's tested and properly licensed.
2.  Since it's only used in one place now, I don't see why we need to generalize it.
3.  The problem boils down to wanting to avoid our legacy system of gsub'ing the version all over the place, and instead just being able to get the version from `'rmagick/version.rb'`.
4. I think a better solution from a design point of view would be to either a) ensure any tasks we run add `lib` to the `$LOAD_PATH` or load the gemspec (`Gem::Specification.load('./rmagick.gemspec').version`) so that we can just `require 'rmagick/version'` and not worry about the relative location of the file