libffi-3.0.11 had been released

About 2 month ago, libffi had been updated to 3.0.11. On MinGW, the DLL file built from libffi source is changed from libffi-5.dll to libffi-6.dll. So that we should rebuild not only libffi-3.0.11 but also any programs using libffi - e.g. Ruby -.