fix ci

.travis.yml

 language: cpp
 cache: ccache
 sudo: required
-dist: xenial 
+dist: Bionic 
 services:
   - docker
 os:
@@ -26,7 +26,7 @@ script:
   - exit_code=0
   - .travis/precommit.sh || exit_code=$(( exit_code | $? ))
   - docker run -i --rm -v "$PWD:/py_unittest" paddlepaddle/paddle:latest /bin/bash -c
-    'cd /py_unittest; sh .travis/unittest.sh' || exit_code=$(( exit_code | $? ))
+    'cd /py_unittest; bash .travis/unittest.sh' || exit_code=$(( exit_code | $? ))
     exit $exit_code
 
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