Remove tokenizer used when validating long strings

See Murray's initial commit message (2b4b7343) for context (2b4b).

Rails 3.2 no longer has performance problems when validating the length
of a very long string.

See https://github.com/rails/rails/commit/5ff71ac9f844f2c8bcde8cdd5de767c9bdd4cc9e#activemodel/lib/active_model/validations/length.rb
Merge pull request #639 from alphagov/remove-validation-tokenizer

Remove tokenizer used when validating long strings