Move to mocha

This moves the tests to `mocha` and `chai`, and also updates the `date-utils` dependency to allow installation of `rel` on windows.

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I know that there was no discussion on this move, but I'd love to see this merged. I'd like to help out working on `rel` (as I'm planning to use it extensively), and having a good test suite available is important to me. I'm planning to port more of the tests from `arel` over to `rel`, and moving to `mocha` is the initial step.