[Truffle] - Using putIfAbsent to add a new RubySymbol in the SymbolsTable.

Thanks @thedarkone to help us with that. Without putIfAbsent multiple
threads might concurrently call createSymbol with the same String name,
create a bunch of RubySymbols, store them in symbolsTable (overwriting
each other) and then return them to rubyland. But using putIfAbsent we
avoid this behaviour.