feat: bump version to 2.5.0

feat: bump version to 2.4.0

feat: bump version to 2.2.0

fix: bump version to 2.1.0

Merge pull request casbin#273 from hackerchai/master

feat: bump version 2.0.9

Merge pull request casbin#271 from hackerchai/version/v2.0.8

feat: bump version 2.0.8

Merge pull request casbin#247 from hackerchai/fix/readme

Fix: fix README.md

Merge pull request casbin#230 from hackerchai/upgrade-tokio

Upgrade tokio

update README.md

upgrade versions of tokio & rhai

Signed-off-by: Cheng JIANG <[email protected]>