* 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

Because travis-ci has been updated Ubuntu Trusty image, it causes
Paddle CI building error. Just using old image now for hot-fix, I
will add another issue to fix Paddle building in new TravisCI image.

Related link
https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch