Do not use a fixed clippy version

Clippy only guarantees, that the latest clippy version will work with
the latest nightly build of Rust.  So we want to always use the latest
version.

Author: rnestler

Cargo.toml

@@ -37,7 +37,7 @@ router = "^0.5"
 error-type = "^0.1"
 serde_json = "^0.8"
 spaceapi = "^0.4.2"
-clippy = {version = "^0.0.87", optional = true}
+clippy = {version = "^0.0", optional = true}
 
 [features]
 unstable = ["clippy"]