Log Message:
Update ruby-ffi to 1.9.18.

* Add AbstractMemory#get and AbstractMemory#put
* Add Pointer#to_ptr
* Update x86_64-openbsd types close #535
* Use latest rake-compiler-dock and rake-compiler.
* Avoid undefined behaviour of LoadLibrary() on Windows.
* Suppress deprecated Fixnum warnings on Ruby 2.4.0.