Upgrade protobuf's version to 3.6.1.

Author: zhouzf05

setup.py

@@ -31,7 +31,7 @@
  long_description=readme,
  packages=['tablestore', 'tablestore.protobuf', 'tablestore.plainbuffer'],
  package_dir={'tablestore.protobuf': 'tablestore/protobuf/' + base_dir},
- install_requires=['enum34', 'protobuf', 'urllib3>=1.14', 'certifi>=2016.2.28', 'future>=0.16.0', 'six>=1.11.0'],
+ install_requires=['enum34', 'protobuf>=3.6.1', 'urllib3>=1.14', 'certifi>=2016.2.28', 'future>=0.16.0', 'six>=1.11.0'],
  include_package_data=True,
  url='https://cn.aliyun.com/product/ots',
  classifiers=[