Use the rake extension for remaining files, for consistency

diff --git a/lib/tasks/subscribe_diary_authors.rb b/lib/tasks/subscribe_diary_authors.rake
similarity index 100%
rename from lib/tasks/subscribe_diary_authors.rb
rename to lib/tasks/subscribe_diary_authors.rake

diff --git a/lib/tasks/subscribe_old_changesets.rb b/lib/tasks/subscribe_old_changesets.rake
similarity index 100%
rename from lib/tasks/subscribe_old_changesets.rb
rename to lib/tasks/subscribe_old_changesets.rake