Compatibility with GHC 7.8

Hi,

The `Foreign` module of 7.8 no longer exports `unsafeForeignPtrToPtr`; it is hidden away in `Foreign.ForeignPtr.Unsafe`.  There was a warning in 7.6.3, but as of 7.8 it's gone.  This is a pretty straightforward fix/patch to maintain compatibility with both 7.6.3 and 7.8.