Also stub the building of the index

Since we don't actually use the index in getting the related posts from
the tests there's no need to build an index, which can take a long time
if the ruby bindings for the GSL library are not installed.

test/test_related_posts.rb

@@ -33,6 +33,7 @@ class TestRelatedPosts < Test::Unit::TestCase
       require 'classifier'
       any_instance_of(::Classifier::LSI) do |c|
         stub(c).find_related { @site.posts[-1..-9] }
+        stub(c).build_index
       end
       assert_equal @site.posts[-1..-9], Jekyll::RelatedPosts.new(@site.posts.last).build
     end