travis: Model via stages

This avoids building the docker images for each Python version test.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.travis.yml

@@ -21,10 +21,14 @@ install:
 script:
   - scripts/checkcode.sh .
   - pytest
+
+jobs:
+  include:
+    - stage: docker
+      python: "3.6"
+      script:
         - docker build -t kasproject/kas .
         - docker build -t kasproject/kas-isar -f Dockerfile.isar .
-
-after_success:
         - if [ "$TRAVIS_BRANCH" == "master" ] || [ "$TRAVIS_BRANCH" == "next" ]; then
               docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD";
               for IMAGE in kasproject/kas kasproject/kas-isar; do