fix memory leak in predict + predict_probability

So I wondered why my app wants 5GB of memory after ~100 training/evaluate-model cycles with a rather small number of trainings examples(3000). I started to investigate...

After eliminating my own code and libsvm itself I had an idea that it could be the wiring inside the rb-libsvm C-extension. Unfortunately Valgrind isn't much use in finding memory leak inside ruby or a ruby C-extension so I had to fall back to simple logging memory usage at specific codepoints. With success.
I could pinpoint it down to the evaluation of a libsvm model which meant a lot of calls to predict_probabilities. A quick exchange of predict_probabilities with just predict made sure that the leak had to be in both methods.

Long Story short the call to `example_to_internal` allocates memory for the nodes which later is never freed. Which mean if you call predict/predict_probablity often enough (30k) you end up with a quite a loot of wasted memory.

Apart from fixing the leak this should not interfere with anything else.