fix missing shakespeare dependency

as suggested by the error message:

  Utility/Yesod.hs:36:8:
      Could not find module `Text.Hamlet'
      It is a member of the hidden package `shakespeare-2.0.0.1'.
      Perhaps you need to add `shakespeare' to the build-depends in your .cabal file.
      Use -v to see a list of the files searched for.