Update to bevy 0.15

[benfrankel]

Fixes #320.

We'll probably want to set up a tag / versioning system soon.

The ChildBuild workaround I chose is probably not ideal, but it works.

Confirmed that the game compiles, runs, and seems fine.

[bas-ie]

Also checked and working here! No idea what to say about the ChildBuild issue, but guess it doesn't block a merge?

[bas-ie]

BTW I think the CI failure here is caused by dynamic_linking no longer working with cargo test --doc, have just been encountering this in another third-party crate.

[alec-deason]

I think this error can be fixed by adding export LD_LIBRARY_PATH=`rustc --print target-libdir` or the equivalent to be CI job before cargo test runs.