[Travis] Limit the amount of docker logs we output on failures

On jobs with debug mode on and whole test suites run it will flood the travis outoput so it never completes.

.travis.yml

@@ -46,8 +46,10 @@ before_script:
 script: docker-compose exec --user www-data app sh -c "php /scripts/wait_for_db.php; php $TEST_CMD"
 
 after_failure:
-  # Will show us the full log of container's main processes (not counting shell process above running php and behat)
-  - docker-compose logs -t --tail="all"
+  # Will show us the last bit of the log of container's main processes
+  # (not counting shell process above running php and behat)
+  # NOTE: errors during docker setup of travis build won't show up here (can't output all as it is too much in debug/verbose mode)
+  - docker-compose logs -t --tail=15
   # Will show us what is up, and how long it's been up
   - docker ps -s