Bump minimum ffi version

On ruby `ruby 1.9.3p385` I experienced following error when installing ffi gem with `1.0.6` minimum version:

```
StructLayout.c:363:9: error: expression is not assignable
        Data_Get_Struct(rbField, StructField, field = layout->fields[i]);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/ruby/1.9.3-p385/include/ruby-1.9.1/ruby/ruby.h:837:12: note: expanded from macro 'Data_Get_Struct'
    (sval) = (type*)DATA_PTR(obj);\
    ~~~~~~ ^
1 error generated.
make: *** [StructLayout.o] Error 1
```

Please allow bumping minimum patch version so it'll compile.