Test beta and nightly too

[Nemo157]

bors r+

[bors[bot]]

Build succeeded:

• cargo clippy
• cargo build -z minimal-versions
• cargo test (beta, x86_64-unknown-linux-gnu, ubuntu-latest)
• cargo test (stable, x86_64-unknown-linux-gnu, ubuntu-latest)
• cargo fmt --check
• cargo test (stable, x86_64-pc-windows-msvc, windows-latest)
• cargo build --update
• cargo test (stable, i686-unknown-linux-gnu, ubuntu-latest)
• cargo test (stable, x86_64-apple-darwin, macos-latest)
• cargo build --target wasm32-unknown-unknown
• cargo test (nightly, x86_64-unknown-linux-gnu, ubuntu-latest)
• cargo test (stable, i686-pc-windows-msvc, windows-latest)