Fix to compile with pipes-3.3.0.

This helped:

http://www.haskellforall.com/2013/05/pipes-330-folds-and-uniting-listt-with.html

Quote from the near the end of the blog post:

    Also, since folds don't use the base monad you no longer need to hoist
    stages that you compose with a fold. For example, if you want to sum
    the first ten lines of user input, you can just write:

    runProxy $ execWriterK $ readLnS >-> takeB_ 10 >-> sumD