cargo: relax stable dependency restrictions

Cargo.toml

@@ -22,7 +22,7 @@ reqwest = "~0.4"
 serde = "~0.9"
 serde_derive = "~0.9"
 serde_json = "~0.9"
-url = "~1.4"
+url = "^1.4"
 
 [features]
 strict = []