added missing -y flag

Author: holgerbrandl

.travis.yml

@@ -23,7 +23,7 @@ install:
 #  - echo "oracle-java8-installer shared/accepted-oracle-license-v1-1 select true" | sudo debconf-set-selections
 #  - sudo apt-get install -y oracle-java8-installer
 
-  - sudo add-apt-repository ppa:openjdk-r/ppa
+  - sudo add-apt-repository -y ppa:openjdk-r/ppa
   - sudo apt-get update
   - sudo apt-get install openjdk-8-jdk