Log Message:
Update ruby-ffi to 1.4.0.

No official changes are available but here is some of them from commit log
on github.

* Add tests for block-form of Buffer::new and MemoryPointer::new
* spec: Test absolute library path.
* DynamicLibrary.c: Avoiding dlsym(dlopen(0,0), sym).
* spec: Replace absolute_.. with expand_path.
* Use rb_thread_call_without_gvl where available.
* Trap & save exceptions from ruby callbacks.