Upgrade `tarpaulin` to 0.21.0 to work around what looks like a bug in coverage report.

Fixes #55

Enable minimal dependencies with new features `bevu_sprite` and
`bevy_ui`, removing the `bevy/render` mandatory dependency. Those new
features are enabled by default, and enable the built-in lenses for the
related Bevy crates. The core `bevy_tweening` crate itself does not take
any optional Bevy dependency anymore, allowing for a slim build with
only the core Bevy functionalities.

Add code coverage via coveralls.io

Bevy 0.6.1 was released which fixes nightly builds. Re-add nightly
toolchain to CI and remove the disclaimer from the README.

Bevy 0.6.0 is broken for nightly, which breaks all `docs.rs` builds.
Remove nightly CI and add a README warning until this is fixed in 0.6.1.