Replace `scala-mode2` with the `ensime` package (#1001)

`scala-mode2` is no longer available on MELPA and it's not maintained anymore.
* the `ensime` package depends on `scala-mode` and `sbt-mode`.

modules/prelude-scala.el

@@ -33,10 +33,10 @@
 ;;; Code:
 
 (require 'prelude-programming)
-(prelude-require-packages '(scala-mode2))
+(prelude-require-packages '(ensime))
 
 (defun prelude-scala-mode-defaults ()
-  (subword-mode +1))
+  (ensime-mode))
 
 (setq prelude-scala-mode-hook 'prelude-scala-mode-defaults)