prediction for new texts broken - `_json_to_instance` uses dataset_reader

[kleinay]

When trying to use allennlp predict for parsing SDP on new texts, it raises an error. The reason is that in utils\transition_sdp_predictor.py, the SDPParserPredictor._json_to_instance method violating the documented contract: Expects JSON that looks like {"sentence": "..."}. The method instead assumes an MRP-formatted input file.