Make the scala-mode dependency explicit

This commit also adds back the auto-enabling of subword-mode together
with scala-mode.

modules/prelude-scala.el

@@ -33,10 +33,11 @@
 ;;; Code:
 
 (require 'prelude-programming)
-(prelude-require-packages '(ensime))
+(prelude-require-packages '(scala-mode ensime))
 
 (defun prelude-scala-mode-defaults ()
-  (ensime-mode))
+  (subword-mode +1)
+  (ensime-mode +1))
 
 (setq prelude-scala-mode-hook 'prelude-scala-mode-defaults)