Add Rails 4 support

Quite a few things needed to be addressed to make this happen:

* Bump AR version in gemspec
* Use Ruby's 1.9+ MiniTest instead of TestUnit.
* [This test](https://github.com/vigetlabs/serialize_with_options/blob/d792a472431c72b93dd00b14e7654a8f2c7ebb89/test/serialize_with_options_test.rb#L184) was failing due to the `except` option [here](https://github.com/vigetlabs/serialize_with_options/blob/master/test/serialize_with_options_test.rb#L12). "email" was replaced with "name".
* The [`encode` method within ActiveSupport](https://github.com/rails/rails/blob/4-1-6/activesupport/lib/active_support/json/encoding.rb#L34) calls `dup` on `options`. When using `to_json`, we were allowing the Symbol for `set` to be passed to this method. `dup` cannot be called on Symbol, so `to_json` had to be added back as a method that we parse the options for.