Limit recently_revised and some lazy evaluations

recently_revised now returns at most the last 50 revised pages.
Also, do some lazy evaluations in the PageSet model (requires
Ruby 2.x, but we're not supporting anything less than 2.0 now).

app/controllers/wiki_controller.rb

app/models/page_set.rb

test/functional/wiki_controller_test.rb

test/test_helper.rb