Add "travis_retry" to our build to reduce transitory failures

Documentation is at https://docs.travis-ci.com/user/common-build-problems/#travis_retry

See docker-library/memcached#14 for the test PR for this pattern.

Author: tianon

.travis.yml

@@ -11,7 +11,7 @@ before_script:
 script:
   - ./update.sh
   - test -z "$(git status --porcelain '*/hello' '*/nanoserver/hello.txt')"
-  - docker build -t "$image" hello-world
+  - travis_retry travis_retry travis_retry docker build -t "$image" hello-world
   - ~/official-images/test/run.sh "$image"
 
 after_script: