Merge branch 'feature/rhel6' into 'master'

Drop CentOS 6

See merge request !16

.gitlab-ci.yml

@@ -85,23 +85,6 @@ upload/epel/7:
   dependencies:
     - build/centos/7
 
-build/centos/6:
-  <<: *build
-  variables:
-    DOCKER_IMAGE: centos/6
-
-test/centos/6:
-  <<: *test
-  variables:
-    DOCKER_IMAGE: centos/6
-  dependencies:
-    - build/centos/6
-
-upload/epel/6:
-  <<: *upload
-  dependencies:
-    - build/centos/6
-
 ###################################
 # Fedora
 ###################################