PyPIRepository: Clean-up wheel cache when done
PyPIRepository: Clean-up wheel cache when done

With Pip 10 initializing a WheelCache will create a temporary directory,
which should be cleaned with its `cleanup` method.  Do this by calling
`wheel_cache.cleanup()` when such method exists (i.e. with Pip 10).

suutari/prequ,suutari/prequ,suutari-ai/prequ