Upgrade rspotify to 0.11.6

Passing IDs to rspotify still feels a little wonky with their `Id` changes
making use of `Cow`. Maybe it can be simplified on our end, but it's a start.

Fixes #844

See also:

https://github.com/ramsayleung/rspotify/pull/332

src/model/playable.rs

@@ -141,6 +141,19 @@ impl From<&PlayableItem> for Playable {
     }
 }
 
+impl From<&Playable> for rspotify::prelude::PlayableId<'_> {
+    fn from(p: &Playable) -> Self {
+        match p {
+            Playable::Track(t) => rspotify::prelude::PlayableId::Track(
+                rspotify::model::TrackId::from_id(t.id.clone().unwrap()).unwrap(),
+            ),
+            Playable::Episode(e) => rspotify::prelude::PlayableId::Episode(
+                rspotify::model::EpisodeId::from_id(e.id.clone()).unwrap(),
+            ),
+        }
+    }
+}
+
 impl fmt::Display for Playable {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {