Use require over require_relative for cross-lib calls

Avoid using `require_relative` with relative path to
lib. Use `require` instead.

More details about this cop can be found at:
https://docs.rubocop.org/rubocop-packaging/cops_packaging.html#packagingrequirerelativehardcodinglib

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>

tasks/documentation.rb

@@ -1,4 +1,4 @@
-require_relative '../lib/shoulda/matchers/version'
+require 'shoulda/matchers/version'
 require 'erb'
 
 module Shoulda