Update tokio requirement from ~0.2 to ~1.8

Updates the requirements on [tokio](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/compare/tokio-0.2.0...tokio-1.8.0)

---
updated-dependencies:
- dependency-name: tokio
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

public-traits/Cargo.toml

@@ -16,7 +16,7 @@ syncable = ["tolstoy_traits", "hyper", "serde_json"]
 failure = "~0.1"
 failure_derive = "~0.1"
 http = "~0.2"
-tokio = { version = "~0.2", features = ["rt-core"] }
+tokio = { version = "~1.8", features = ["rt-core"] }
 uuid = "~0.8"
 
 [dependencies.rusqlite]

tolstoy/Cargo.toml

@@ -16,7 +16,7 @@ hyper-tls = "~0.4"
 http = "~0.2"
 log = "~0.4"
 mime = "~0.3"
-tokio = { version = "~0.2", features = ["full"] }
+tokio = { version = "~1.8", features = ["full"] }
 serde = "~1.0"
 serde_json = "~1.0"
 serde_cbor = "~0.11"