Build Cache for Gradle v2.20.9

Changelog
1

What’s Changed

  • Bump bitrise-build-cache-cli to v2.8.4. Headline: config-cache keys now sanitize / in git branch names, so branches like feature/foo no longer collide when building config-cache keys (cli#353).
  • Also picks up intervening CLI versions v2.8.2–v2.8.3 (release-infrastructure changes only: R2 mirror of release artifacts + automated preboot startup-script bump — no Gradle-side behavior changes).