build: auto accept repository addition (#4480)

.travis.yml

@@ -16,7 +16,7 @@ env:
 before_install:
   # upgrade docker to the latest supported by the OS loaded in the travis image
   - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-  - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+  - sudo add-apt-repository --yes "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
   - sudo apt-get update
   - sudo apt-get -y -o Dpkg::Options::="--force-confnew" install docker-ce