test=develop

* Simplify the travis script.
Now use docker to deploy documentations

* Check for the pull request

* Update paddle_build.sh

* Update paddle_build.sh

release branch in the futrue

* Build: simplify travis CI script.

* Add linkcheck and run gen_doc in host machine not in docker.

* Add LinkCheck python package.

* Refine CheckStyle Script

* Disable linkchecker for build_doc.sh

It seems that Travis change the running image. The /usr/local/ is
not writable in new images. Use `sudo` for pip to resolve this.

Remove build_android task from travis temporarily to avoid failure on travis because of the cache of third_party.

Fix issue #2562