Add a retry on docker pull to be more resilient to intermittent failures (#4609)

diff --git a/.travis/install.sh b/.travis/install.sh
index def7343..a4bc3c9 100755
--- a/.travis/install.sh
+++ b/.travis/install.sh
@@ -51,7 +51,7 @@
         echo "OPENSSL and LIBRESSL are not allowed when DOCKER is set."
         exit 1
     fi
-    docker pull "$DOCKER"
+    docker pull "$DOCKER" || docker pull "$DOCKER" || docker pull "$DOCKER"
 fi
 
 if [ -z "${DOWNSTREAM}" ]; then