Use direct path to rake file
Use direct path to rake file

Before this, we were relying on the Gem's `lib` dir being on the
$LOAD_PATH, and thus `load` would find the `tasks/doc.rake` file. This
can break when a different gem also has a `tasks/docs.rake` file, and
it's further up in the load path. Same goes for a Rails app with a
`tasks/docs.rake` file existing. This ensures we get OUR
`tasks/docs.rake` file.

zipmark/rspec_api_documentation,zipmark/rspec_api_documentation,zipmark/rspec_api_documentation