Using previous image for travis-ci

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

.travis.yml

+group: deprecated-2017Q2
 language: cpp
 cache:
   directories: