C extension support on rubinius with ffi gem
C extension support on rubinius with ffi gem

Rubinius ships with an FFI implementation but it is missing
FFI::ManagedStruct. This change documents the reason and switches to
requiring the ffi gem under Rubinius. In fact, now all rubies run
require 'ffi'.

OpenBEL/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb,OpenBEL/bel.rb,abargnesi/bel.rb