Reorganize gem to compile with ruby2

I'v tested with Ruby 1.9.3 and Ruby 2.0.0 and compile and install without problem.

You could also try it adding to a Gemfile : 

<pre>
gem 'geoip-c', require: "geoip", git: "git://github.com/Gazer/geoip.git", branch: "ruby2"
</pre>