Inform that Emacs 24 is a requirement to install Cask.

rejeep · rejeep · commit cb18b202ed3b · 2014-05-23T11:46:23.000+02:00
diff --git a/02-installation.md b/02-installation.md
@@ -6,6 +6,8 @@ permalink: installation.html
 
 ## Installation
 
+To install Cask you need Emacs 24 or higher.
+
 ---
 
 To automatically install Cask, run this command:
