Remove use of `pp` in DifferDispatcher

[sidane]

Was causing the following output when running tests in Rails app using super_diff:

{:available_classes=>
  [SuperDiff::ActiveRecord::Differs::ActiveRecordRelation,
   SuperDiff::ActiveSupport::Differs::HashWithIndifferentAccess,
   SuperDiff::RSpec::Differs::CollectionContainingExactly,
   SuperDiff::RSpec::Differs::CollectionIncluding,
   SuperDiff::RSpec::Differs::HashIncluding,
   SuperDiff::RSpec::Differs::ObjectHavingAttributes,
   SuperDiff::Basic::Differs::Array,
   SuperDiff::Basic::Differs::Hash,
   SuperDiff::Basic::Differs::TimeLike,
   SuperDiff::Basic::Differs::DateLike,
   SuperDiff::Basic::Differs::MultilineString,
   SuperDiff::Basic::Differs::CustomObject,
   SuperDiff::Basic::Differs::DefaultObject]}

[mcmire]

Oops! Thank you :)