fix broken cask list

lib/cask/scopes.rb

@@ -23,7 +23,7 @@ module Cask::Scopes
     end
 
     def installed
-      all.select { |c| c.installed? }
+      all_titles.select { |c| Cask.load(c).installed? }
     end
   end
 end

test/cask/scopes_test.rb

@@ -0,0 +1,19 @@
+require 'test_helper'
+
+describe Cask::Scopes do
+  describe 'installed' do
+    it "returns a list of strings" do 
+      caffeine = Cask.load('caffeine')
+
+      shutup do
+        Cask::Installer.install(caffeine)
+      end
+
+      installed_casks = Cask.installed
+      installed_casks.count.must_be :>, 0
+      installed_casks.each do |c|
+        c.must_be_kind_of String
+      end
+    end
+  end
+end