Rails 4.0 support

Added Rails 4.0 support to record-cache, see #38.

**Note**: there some breaking changes in the specs (due to changes in Rails). Because of this, most specs will fails on Rails versions < 4. I suggest moving development on the Rails 3.x versions on a branch before merging this into master.

Summary of biggest changes:
* Replaced the after commit patch by the test_after_commit gem.
* Added new datastore for Rails 4.0. 
* Fixed call to unserialize (in `lib/record_cache/strategy/util.rb`).
* Fixed deprecation of `Foo.where(bar: :baz).all` calls. You have to use either `#load` (if you only want to eagerly load the relation) or `#to_a` (if you want to convert to an array). This change only affected the specs.

I will continue to work on Rails 4.1 and 4.2 support later.