add test to travis to check if all images in compose can be pulled from docker hub (and can also be started)

Signed-off-by: Felix Bartels <felix@host-consultants.de>

.travis.yml

@@ -36,3 +36,8 @@ deploy:
     script: make publish
     on:
       branch: master
+after_deploy:
+  - provider: script
+    script: docker system prune -a -f && make test-ci && echo "LDAP_CONTAINER=kopano_ldap" >> .env && make test-ci
+    on:
+      branch: master