Warn when overriding ScriptFileFormula#install

ScriptFileFormula's use is that it installs whatever was downloaded
to bin; if the install is overridden, there's no benefit over deriving
directly from Formula.

Library/Homebrew/formula_specialties.rb

@@ -5,6 +5,14 @@ class ScriptFileFormula < Formula
   def install
     bin.install Dir['*']
   end
+
+  def self.method_added method
+    super method
+    case method
+    when :install
+      opoo "#{name}: if you are overriding ScriptFileFormula#install, use a Formula instead"
+    end
+  end
 end
 
 # See browser for an example