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

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.

Fixes #1365