Updated settings::utils::write_file’s signature for more flexibility

`write_file` now accepts as its second argument any type that
implements the `ToString` trait so that not only Strings will be
accepted but any type that can be turned into one.

src/settings/mod.rs

@@ -188,7 +188,7 @@ fn write_settings() {
 version: "1"
 categories: []
 rules: []"#;
-    utils::write_file(&path, serde_yaml::to_string(&s).unwrap()).unwrap();
+    utils::write_file(&path, &serde_yaml::to_string(&s).unwrap()).unwrap();
     assert_eq!(settings, utils::read_file(&path).unwrap());
 }