update dockerfile: remove hmm_treebank_pos_tagger because of building error as below.


[nltk_data] Error loading hmm_treebank_pos_tagger: Package [nltk_data] 'hmm_treebank_pos_tagger' not found in index Error installing package. Retry? [n/y/e] �[91m/opt/conda/lib/python3.6/runpy.py:125: RuntimeWarning: 'nltk.downloader' found in sys.modules after import of package 'nltk', but prior to execution of 'nltk.downloader'; this may result in unpredictable behaviour warn(RuntimeWarning(msg)) �[0m �[91mTraceback (most recent call last): File "/opt/conda/lib/python3.6/runpy.py", line 193, in _run_module_as_main "__main__", mod_spec) File "/opt/conda/lib/python3.6/runpy.py", line 85, in _run_code exec(code, run_globals) File "/opt/conda/lib/python3.6/site-packages/nltk/downloader.py", line 2271, in <module> halt_on_error=options.halt_on_error) File "/opt/conda/lib/python3.6/site-packages/nltk/downloader.py", line 681, in download choice = input().strip() EOFError: EOF when reading a line �[0m Removing intermediate container 554efc8886c1