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.

lib/rspec_api_documentation/railtie.rb

@@ -1,7 +1,7 @@
 module RspecApiDocumentation
   class Railtie < Rails::Railtie
     rake_tasks do
-      load "tasks/docs.rake"
+      load File.join(File.dirname(__FILE__), '../tasks/docs.rake')
     end
   end
 end