fix failing Ruby tests on 64-bit Linux

This PR fixes some of the failing tests in the Ruby bindings. Note that this is only fixing the current problems, marking the tip of the iceberg... 
IMHO Ruby-DL, the FFI used in the Gt Ruby bindings, is hopelessly outdated -- it is limited to Ruby 1.8, can not handle 64-bit integers as immediate values and only allows for a hard-coded, fixed number of C callbacks at once (usually 10) which we need to use in many places, custom visitors/streams being the most prominent. I think we should sooner or later say goodbye to this FFI and look for something new, e.g. https://github.com/ffi/ffi or something else altogether.