Prefer ERb over Erubis as default engine for .erb files

Erubis is the preferred choice of Tilt.
Actually, if a library built on top of Tilt wants to reuse the default mapping, is forced to have it as dependency.

To be clear, I don't have nothing against Erubis, but it's an external dependency, and in general, reduce dependencies is always good for end developers. Let them to opt-in for it, if they prefer.