Tab#for_formula: using Formula#installed_prefix

Closes #529.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/tab.rb

@@ -109,7 +109,7 @@ class Tab < OpenStruct
       paths << dirs.first
     end
 
-    paths << f.prefix
+    paths << f.installed_prefix
 
     path = paths.map { |pn| pn.join(FILENAME) }.find(&:file?)