install gcc-c++ with -y

Former-commit-id: 803e76802e2302cc96dd41451175fca599876f4d

.gitlab-ci.yml

@@ -9,7 +9,7 @@ test-centos7:
   - yum install install -y python34-pip python34-devel
   - yum install -y gcc graphviz graphviz-devel
   - yum install -y httpd httpd-devel
-  - yum install gcc-c++
+  - yum install -y gcc-c++
   - cd ippisite
   - pip3 install -r requirements.txt
   - python3 manage.py test