Update test_sentencepiece.model url (#279)

Author: gpengzhi

texar/torch/data/tokenizers/xlnet_tokenizer_test.py

@@ -20,7 +20,7 @@ def setUp(self):
         self.tmp_dir = tempfile.TemporaryDirectory()
         self.SAMPLE_VOCAB = maybe_download(
             'https://github.com/huggingface/transformers/blob/master/'
-            'transformers/tests/fixtures/test_sentencepiece.model?raw=true',
+            'tests/fixtures/test_sentencepiece.model?raw=true',
             self.tmp_dir.name)
 
         self.tokenizer = XLNetTokenizer.load(