add a retry on travis builds

Author: JoshKarpel

.travis.yml

@@ -15,7 +15,7 @@ matrix:
   fast_finish: true
 
 install:
-  - docker build -t dags-test --file tests/_inf/Dockerfile --build-arg HTCONDOR_VERSION --build-arg PYTHON_VERSION=$TRAVIS_PYTHON_VERSION .
+  - travis_retry docker build -t dags-test --file tests/_inf/Dockerfile --build-arg HTCONDOR_VERSION --build-arg PYTHON_VERSION=$TRAVIS_PYTHON_VERSION .
 
 script:
   - docker run dags-test tests/_inf/travis.sh