fix(travis-ci): update logger makefile target to "test-unit"

The "make test" target in logger now runs functional tests as well,
requiring Docker. This was not the intention at Travis-ci.org, where
we want just unit tests.

Author: mboersma

.travis.yml

@@ -41,7 +41,7 @@ script:
   - make -C controller flake8
   - make -C controller coverage
   - make -C docs
-  - make -C logger test
+  - make -C logger test-unit
 
 after_success:
   - cd controller && coveralls