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

* install wheels when invoke paddle script if current python don't
  have paddle packages, or installed a previous version.
* Also add `make install` to travis