Use Ruby's require_relative to load project specific libraries.

By using require_relative, we can run cpc and spec tests without
having to be in a certain directory.  The require_relative method
was added in Ruby 1.9.2.  For 1.8.x compatibility, the backports gem
featuring the same require_relative functionality has been added and
will be loaded if necessary.

See https://practicingruby.com/articles/ways-to-load-code?u=dc2ab0f9bb