Tab: handle non-core kegs without receipts

Passing Formula.factory the name of a keg that belongs to a non-core
formula will cause an error to be raised; we don't really care, so just
fake a totally empty install receipt in this case.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/tab.rb

@@ -33,7 +33,11 @@ class Tab < OpenStruct
     if path.exist?
       self.from_file path
     else
-      self.dummy_tab Formula.factory(keg.parent.basename)
+      begin
+        self.dummy_tab Formula.factory(keg.parent.basename)
+      rescue FormulaUnavailableError
+        Tab.new :used_options => [], :unused_options => []
+      end
     end
   end