using xenial dist ci

.travis.yml

 language: cpp
 cache: ccache
 sudo: required
-dist: trusty
+dist: xenial 
 services:
   - docker
 os:
@@ -13,12 +13,10 @@ addons:
   apt:
     packages:
       - git
-      #- python3.5
-      #- python3-dev
 
 before_install:
   -  python3 --version
-  #-  sudo python3 -m pip install -U virtualenv pre-commit pip
+  -  pip3 --version
   -  pip3 install -U virtualenv pre-commit pip
   -  docker pull paddlepaddle/paddle:latest