Fix compatibility with newer Haskell libraries

This small patch fixes compatibility with a few newer Haskell libraries:

- base 4.6, included with ghc 7.6, removed the deprecated 'catch'
  function from Prelude, so our "import Prelude hiding (catch)" is now
  an error; we workaround by using fully-qualified
  Control.Exception.catch name

- containers 0.5 changed the signature of 'deleteFindMax'; we
  workaround by using separate 'findMax' and 'deleteMax'

- QuickCheck 2.5 removed the 'maxDiscards' test parameter, replacing
  it with a much better 'maxDiscardsRatio'; however, until we can
  depend on that, we workaround by just removing it (we don't control
  anymore the maxDiscards, instead leaving it default; for our default
  test size, this is no change, as the default value is already 500,
  which is our default as well) and not printing it anymore

Tested on Squeeze (+extra libs), Wheezy and experimental, which covers
all supported GHC versions.

Also, merging this in master will be a pain, but unless we want to
stop supporting 2.6…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>