Added .kv to auto install list

Kivy is a python framework for building desktop and android apps, it has a specific language called Kv language and files of this language should have `.kv` extension.

core/prelude-packages.el

@@ -143,6 +143,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.groovy\\'" groovy-mode groovy-mode)
     ("\\.haml\\'" haml-mode haml-mode)
     ("\\.hs\\'" haskell-mode haskell-mode)
+    ("\\.kv\\'" kivy-mode kivy-mode)
     ("\\.latex\\'" auctex LaTeX-mode)
     ("\\.less\\'" less-css-mode less-css-mode)
     ("\\.lua\\'" lua-mode lua-mode)