fix ci.

.travis.yml

@@ -16,11 +16,12 @@ addons:
       - python
       - python-pip
       - python2.7-dev
+      - clang-format-3.9
   ssh_known_hosts: 52.76.173.135
-
 before_install:
-  -  sudo pip install -U virtualenv pre-commit pip
-  -  docker pull paddlepaddle/paddle:latest
+  - if [[ "$JOB" == "PRE_COMMIT" ]]; then sudo ln -s /usr/bin/clang-format-3.9 /usr/bin/clang-format; fi
+  - sudo pip install -U virtualenv pre-commit pip
+  - docker pull paddlepaddle/paddle:latest
 
 script:
   - exit_code=0