version to 0.21.0

* Use Ardb for setting up ActiveRecord in the test suite (#124)
* Remove shared support models for unit tests (#125)
* Use Ardb's `RelationSpy` (#127)
* Redesign test DB schema (#128)
* Use the test DB schema and setup in bench scripts (#129)
* Expand read model benchmarks (#131)
* Use AR column type casting with read models (#132)
* `MR::Model` stop using the index operator to r/w fields (#135)
* Benchmark factory data generation (#138)
* Commonize AR type-casting logic (#139)
* Use AR type-casting with `MR::Factory` (#140)
* Update to support rbx 2.2.1 and MRI 2.0.0 (#141)

/cc @kellyredding