add nltk dependency in paddle/setup.py.in

paddle/setup.py.in

@@ -21,6 +21,7 @@ setup(name="py_paddle",
       include_package_data=True,
       package_data={'py_paddle':['*.py','_swig_paddle.so']},
       install_requires = [
+        'nltk>=3.2.2',
         'numpy>=1.8.0',      # The numpy is required.
         'protobuf>=${PROTOBUF_VERSION}'    # The paddle protobuf version
       ],