Auto-install swift-mode when needed

core/prelude-packages.el

@@ -154,6 +154,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.scala\\'" scala-mode2 scala-mode)
     ("\\.scss\\'" scss-mode scss-mode)
     ("\\.slim\\'" slim-mode slim-mode)
+    ("\\.swift\\'" swift-mode swift-mode)
     ("\\.textile\\'" textile-mode textile-mode)
     ("\\.yml\\'" yaml-mode yaml-mode)
     ("Dockerfile\\'" dockerfile-mode dockerfile-mode)))