Add `FlexibleContexts` to fix compilation with GHC HEAD

This fixes the compile error

  tests/builder/Data/ByteString/Builder/Tests.hs:210:5:
    Non type-variable argument
      in the constraint: MonadWriter (D.DList Word8) m
    (Use FlexibleContexts to permit this)
    In the context: (MonadWriter (D.DList Word8) m, MonadState Int m)
    While checking the inferred type for ‘renderAction’
    ...

See also

  http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/24612

for more details