Add kotlin-mode to the auto-install list (#1016)

core/prelude-packages.el

@@ -150,6 +150,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.haml\\'" haml-mode haml-mode)
     ("\\.hs\\'" haskell-mode haskell-mode)
     ("\\.json\\'" json-mode json-mode)
+    ("\\.kt\\'" kotlin-mode kotlin-mode)
     ("\\.kv\\'" kivy-mode kivy-mode)
     ("\\.latex\\'" auctex LaTeX-mode)
     ("\\.less\\'" less-css-mode less-css-mode)