build: fix compilation without default features (#218)

* build: fix compilation with optional features
* Remove unused config from Cargo.toml
* Fix cache keys
Co-authored-by: Luis Moreno <morenol@users.noreply.github.com>

src/svm/svc.rs

@@ -133,7 +133,7 @@ pub struct SVCParameters<
 pub struct SVC<'a, TX: Number + RealNumber, TY: Number + Ord, X: Array2<TX>, Y: Array1<TY>> {
     classes: Option<Vec<TY>>,
     instances: Option<Vec<Vec<TX>>>,
-    #[serde(skip)]
+    #[cfg_attr(feature = "serde", serde(skip))]
     parameters: Option<&'a SVCParameters<'a, TX, TY, X, Y>>,
     w: Option<Vec<TX>>,
     b: Option<TX>,