bump CMake minimum required version to avoid warnings

[alexlnkp]

fixes future deprecation

CMake Deprecation Warning at lib/sentencepiece/CMakeLists.txt:15 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

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[alexlnkp]

I'm using a github provided EMail address for commits, what do you want me to do?