check-exercises: Fix warning with ghc >= 7.10.
check-exercises: Fix warning with ghc >= 7.10.

This fixes the warning "The import of Control.Applicative is redundant"
cause by Prelude re-exporting Control.Applicative.<$> in base >= 4.8.

Removing the offending line is undesirable because it would break
compiling if ghc <= 7.10.

exercism/xhaskell