* `pip install` using `find` utility

* Change build process to generate v2 documentation
* Add directory for v2 api
* Correct sphinx index soft-link

We don't officially support OSX.
And Travis CI takes a lot of time to build osx. (long time in pending state.
Probably because travis ci don't have enough osx machines.

* 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.