Release 2.6.0

[MichaelGHSeg]

Bumps VERSION and dates the existing Unreleased changelog section. No code changes.

Releases the HTTP work already on master:

• Dual-path retry: exponential backoff + rate-limit handling #280 — dual-path retry: exponential backoff + rate-limit handling
• Bound the rate-limit retry budget at 30 minutes #284 — bound the rate-limit retry budget at 30 minutes

Minor rather than patch: new X-Retry-Count request header, and the default backoff schedule changes (base 500ms rather than 100ms, ceiling 60s rather than 10s, multiplier 2 rather than 1.5) — both carry upgrade notes — but nothing is removed from the API.

.travis.yml also contains 2.5.0; that is a Ruby version in the test matrix, not the gem version, and is deliberately untouched.

The RubyGems trusted publisher is registered and matches deploy.yml: repository segmentio/analytics-ruby, workflow deploy.yml, environment production.

After merge: tag 2.6.0 (bare, no v), then create a GitHub Release. deploy.yml triggers on release: published, verifies the tag against version.rb, and publishes via trusted publishing. The production environment requires review from libraries-web-team.