Support ActiveRecord 3.2

This turned out to not require any actual changes.

Tests run clean, except for a warning:

```
λ BUNDLE_GEMFILE=gemfiles/ar-3.2.gemfile bundle exec rake test
/opt/rubies/2.1.2-github1/bin/ruby -I"lib:lib:test" -I"/Users/charlie/github/serialized_attributes/gemfiles/vendor/gems/ruby/2.1.0/gems/rake-10.3.2/lib" "/Users/charlie/github/serialized_attributes/gemfiles/vendor/gems/ruby/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb" "test/**/*_test.rb"
Run options:

# Running tests:

[74/82] SerializedAttributeWithSerializedDataTestWithActiveSupportJson#test_reloads_serialized_dataDEPRECATION WARNING: You're trying to create an attribute `id'. Writing arbitrary attributes on a model is deprecated. Please just use `attr_writer` etc. (called from block (3 levels) in <top (required)> at /Users/charlie/github/serialized_attributes/test/serialized_attributes_test.rb:78)
DEPRECATION WARNING: You're trying to create an attribute `id'. Writing arbitrary attributes on a model is deprecated. Please just use `attr_writer` etc. (called from find at /Users/charlie/github/serialized_attributes/test/test_helper.rb:58)
Finished tests in 0.056702s, 1446.1571 tests/s, 2980.4945 assertions/s.
82 tests, 169 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.1.2p95-github (development) [x86_64-darwin13.0]
```