Reorganize unit tests, part I

* Move spec/shoulda to spec/unit_tests/shoulda
* Move spec/support/*.rb to spec/support/unit_tests/{helpers,matchers}
* Move spec_helper.rb to unit_spec_helper.rb
Reorganize unit tests, part II

* Change 'spec' Rake task to 'spec:unit'
* Require unit_spec_helper.rb in unit tests, not spec_helper.rb
* Re-namespace files in spec/support/unit under UnitTests
* Files in spec/support/unit/helpers no longer automatically add
  themselves to RSpec - this happens in unit_spec_helper.rb
* Extract RecordWithDifferentErrorAttributeBuilder and
  RecordValidatingConfirmationBuilder to separate files
Add new RSpec acceptance tests to replace Cucumber
Remove Cucumber tests
Fix unit tests to use new acceptance stuff
Tell Travis how now to run the tests
Add Ruby 2.1.4 to Travis
Fix deprecation warning on Rails 3.2
Rename {ensure => validate}_length_of
Deprecate ensure_length_of for validate_length_of
Rearrange some things in NEWS