Allow CPC to run from anywhere.

This construction allows CPC to reference the candlepin_api.rb file
without depending on the working directory.  It is compatible with Ruby
1.8.x, 1.9.x, and 2.0.  See http://stackoverflow.com/questions/4325759

In an ideal world, we'd forsake Ruby 1.8.x and just use require_relative which was introduced in 1.9.1; however, it is very nice to be able to run everything in RHEL 6.