Bump version to 2.9.0 (mastodon#11074)

Author: Gargron

CHANGELOG.md

@@ -3,7 +3,7 @@ Changelog
 
 All notable changes to this project will be documented in this file.
 
-## [Unreleased]
+## [2.9.0] - 2019-06-13
 ### Added
 
 - **Add single-column mode in web UI** ([Gargron](https://github.com/tootsuite/mastodon/pull/10807), [Gargron](https://github.com/tootsuite/mastodon/pull/10848), [Gargron](https://github.com/tootsuite/mastodon/pull/11003), [Gargron](https://github.com/tootsuite/mastodon/pull/10961), [Hanage999](https://github.com/tootsuite/mastodon/pull/10915), [noellabo](https://github.com/tootsuite/mastodon/pull/10917), [abcang](https://github.com/tootsuite/mastodon/pull/10859), [Gargron](https://github.com/tootsuite/mastodon/pull/10820), [Gargron](https://github.com/tootsuite/mastodon/pull/10835), [Gargron](https://github.com/tootsuite/mastodon/pull/10809), [Gargron](https://github.com/tootsuite/mastodon/pull/10963), [noellabo](https://github.com/tootsuite/mastodon/pull/10883), [Hanage999](https://github.com/tootsuite/mastodon/pull/10839))

lib/mastodon/version.rb

@@ -21,7 +21,7 @@ def pre
     end
 
     def flags
-      'rc2'
+      ''
     end
 
     def to_a