Fixed the docker-compose testing dependency

Running `make check` breaks right now when the testing dependency
`docker-compose` is installed in the version 1.12.0 (see the following
GitHub issue: docker/compose#4729).

Fixing the dependency to versions below 1.12.0 solves the problems and
`make check` will result in green tests again.

Change-Id: I62d83a1ef4495bae3699c2f89deecf213be82364
Signed-off-by: dpdornseifer <david.paul.dornseifer@sap.com>

requirements-test.txt

@@ -1,6 +1,6 @@
 coverage
-docker-compose >= 1.7.0
+docker-compose >= 1.7.0,<1.12.0
 flake8
 mock
 pytest
-pytest-cov
+pytest-cov