Update Travis CI config, use latest rbx and add ruby 2.1

This updates the Travis CI config to use the latest rbx and also
adds ruby 2.1. These are the current versions being supported by
MR.

/cc @kellyredding
version to 0.23.0

* Remove 'caching' of `Factory` dates and times (#169)
* `FakeRecord`, no longer track attribute changes (#170)
* `RecordStack`, reset fake records `save_called` flag (#171)
* Fix edge case with `assert_not_field_saved` (#172)
* Add `model_reset_save_called` test helper (#173)
* `Factory`, speed up random dates and times generation (#175)

/cc @kellyredding