Update to the last rust-nightly.

Version of rustc: 0.12.0-pre-nightly (aa0e35bc6 2014-07-22 00:26:21 +0000)
Make the deactivation threshold customizable.

This adds a `set_deactivation_threshold` method to the rigid
bodies.  Use it to set the minimum squared velocity that a rigid
body must have not to be deactivable. The default threshold is
`Some(1.0)`.

See the demo `examples/regression_tests/issue_11` for an
example.

Fix #11.