I am getting following error without this commit when doing docker build.

  File "<string>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/setuptools/__init__.py", line 12, in <module>
    import setuptools.version
  File "/usr/local/lib/python2.7/dist-packages/setuptools/version.py", line 1, in <module>
    import pkg_resources
  File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 70, in <module>
    import packaging.version
ImportError: No module named packaging.version

No idea why this change fix the problem, but it does fix.

Add a `paddle usage` command to support collecting demo usage.

* The $TRAVIS_COMMIT_RANGE could be not existed, because the git history
  can be changed and rebased. Here we alway run Travis-CI when `git diff`
  cannot return the modified files.
* Also do not push docs update if is not PaddlePaddle/Paddle.