revert tokio upgrade

Signed-off-by: Cheng JIANG <alex_cj96@foxmail.com>

Cargo.toml

@@ -6,26 +6,26 @@ homepage    = "https://github.com/casbin-rs/diesel-adapter"
 license     = "Apache-2.0"
 name        = "diesel-adapter"
 readme      = "README.md"
-version     = "0.8.2"
+version     = "0.8.3"
 
 [dependencies]
 async-std   = { version = "1.8.0", default-features = false, optional = true }
 async-trait = "0.1.40"
-casbin      = { version = "2.0.4", default-features = false }
+casbin      = { version = "~2.0.5", default-features = false }
 diesel      = { version = "1.4.5", default-features = false, features = ["r2d2"] }
-futures     = "0.3.5"
-tokio       = { version = "0.3.5", default-features = false, optional = true }
+futures     = "~0.3"
+tokio       = { version = "~0.2.22", default-features = false, optional = true }
 
 [features]
-default = ["postgres", "runtime-async-std"]
+default = ["postgres", "runtime-tokio"]
 
 mysql    = ["diesel/mysql"]
 postgres = ["diesel/postgres"]
 sqlite   = ["diesel/sqlite"]
 
 runtime-async-std = ["casbin/runtime-async-std", "async-std/unstable"]
-runtime-tokio     = ["casbin/runtime-tokio", "tokio/rt"]
+runtime-tokio     = ["casbin/runtime-tokio", "tokio/blocking", "tokio/rt-core"]
 
 [dev-dependencies]
 async-std = { version = "1.8.0", features = ["attributes"] }
-tokio     = { version = "0.3.5", features = ["full"] }
+tokio     = { version = "~0.2.22", features = ["full"] }