For commands which do not have a built in retry feature, use the travis_retry function to retry it up three times if the return code is non-zero

Author: hugovk

.travis.yml

@@ -42,7 +42,7 @@ install:
   - if [ "$DOCKER" == "" ]; then .travis/install.sh; fi
 
 before_install:
-  - if [ "$DOCKER" ]; then docker pull pythonpillow/$DOCKER:$DOCKER_TAG; fi
+  - if [ "$DOCKER" ]; then travis_retry docker pull pythonpillow/$DOCKER:$DOCKER_TAG; fi
 
 before_script:
 # Qt needs a display for some of the tests, and it's only run on the system site packages install