Bump the version number to 0.15.

[dgohman-fastly]

In preparation for a release, bump the version number tol 0.15.

[ulyssa]

2025-10-27T17:17:30.1075501Z cd cli/tests/trap-test && RUST_BACKTRACE=1 cargo --locked test fatal_error_traps -- --nocapture
2025-10-27T17:17:30.2317546Z     Updating crates.io index
2025-10-27T17:17:31.2216249Z error: the lock file /home/runner/work/Viceroy/Viceroy/cli/tests/trap-test/Cargo.lock needs to be updated but --locked was passed to prevent this
2025-10-27T17:17:31.2217439Z If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.

Looks like a lockfile needs to be checked in to account for the bump.