(PUP-3479) Update ffi to ~1.9.5

Ruby 2.1.x on Windows requires ffi 1.9.5+. Update the ffi dependency to
1.9.5.
Merge pull request #807 from MikaelSmith/task/master/PUP-3479-update-ffi

(PUP-3479) Update ffi to ~1.9.5