Update Docker image versions

.travis.yml

@@ -9,21 +9,21 @@ env:
     - DISTRIBUTION: ubuntu
       VERSION: 16.04
     - DISTRIBUTION: debian
-      VERSION: 8
+      VERSION: 9
     - DISTRIBUTION: fedora
-      VERSION: 25
+      VERSION: 27
 
 services:
   - docker
 
 before_install:
-    # Install latest Git
+  # Install latest Git
   - sudo apt-get update
   - sudo apt-get install --only-upgrade git
   - sudo apt-get install smbclient
-    # Allow fetching other branches than master
+  # Allow fetching other branches than master
   - git config remote.origin.fetch +refs/heads/*:refs/remotes/origin/*
-    # Fetch the branch with test code
+  # Fetch the branch with test code
   - git fetch origin docker-tests
   - git worktree add docker-tests origin/docker-tests