* 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.

Just forget to add deploy_key to this repository.

to restore successful build status. travis DOCS job on master failed due to permisstion issue: https://travis-ci.org/baidu/Paddle/jobs/159851334  @reyoung, please resolve the permission issue.

* Add documentation build to ci test
* Push master branch doc changes to gh-pages branch