Update for Rails 4.1

This PR contains all the code in #90 but with two minor changes:

* add a `require 'yaml'` in the ActiveYaml base file. All the related tests were failing to locate the `YAML` constant otherwise.

* fixed the Travis config to use 2.1.2, as 2.1.0 and 2.1.1 both had regressions ([2.1.0](https://bugs.ruby-lang.org/issues/9470), [2.1.1](https://www.ruby-lang.org/en/news/2014/03/10/regression-of-hash-reject-in-ruby-2-1-1/)). Updated the config to exclude Rails 2.3 from testing against newer Rubies.

May want to hold off merging for another couple days; I'll be trying this version out against a real application upgrading to 4.1 next week.